« PrejšnjaNaprej »
may be warmed by the sun while the other two sides are cooled by a cold wind. The occupants of rooms on these sides may require air ten degrees higher in
temperature. The individual duct system allows them to get it. The direct radiators placed in the classrooms cannot be depended upon to maintain the proper balance when a trunk system is used and air of the same temperature supplied to every room. The sunny
side rooms need cooler air, and cannot get it by the trunk system. The individual duct system, moreover, is excellent in allowing monotony of atmospheric conditions to be avoided; a cooler or a warmer air may be had at will.
Mr. Kimball says: "The installation of the individual duct system increases the cost of the ventilating plant by two and one half per cent. in large, and five per cent. in small buildings." It is well worth it. He says automatic temperature-controlling systems may save ten to twenty per cent. of the annual fuel bill, and protect the class-rooms from excessive temperatures. By thermostatic devices in mild weather little or no steam is admitted to the class-room radiators, and cool air is delivered. In cold weather much steam is sent into the radiators and a warmer air delivered. Mr. Kimball says a thorough diffusion of the air through every portion of a class-room can be obtained by one or two fresh-air openings eight feet above the floor, with a single exhaust opening on the same side at the floor. The inlets should be directed towards the windows or be placed in the end walls as near as possible to the windows, and the outlet at the other end of the same wall near the floor. In churches and auditoriums the best results can be obtained by exhausting through mushroom openings near the floor and supplying air through openings in the walls or ceiling. The cool incoming air must be let in well above the occupants, so as not to cause unpleasant draughts.
În warm weather it is desirable to be able to reverse the current and send the cool air in at the floor. It is quite easy to arrange for this in the plant. Better still, both the in- and out-fans may be altered so as to impel into the building air which escapes through open doors and windows-an admirable plan. The cost of the plant such as sketched above is reckoned as about one and a halfpenny per cubic foot of space in the building. Only a proper combination of fresh-air supply and vitiated-air exhaust will afford a good ventilation in an auditorium. No occupant should be further than twenty-five feet from a freshair register or nearer than six feet to a vent register; the greater the number of both kinds of registers, the greater the satisfaction with the system. Direct radiators should be employed to balance loss of heat through windows and walls. They should be placed under windows, by doors, and along exterior walls. A large amount of radiation should be placed in vestibules to prevent annoying draughts.
In the temperate climate of Britain we can make do with an open fire and window system, which is intolerable in places where excessively low winter temperatures have to be faced. As, however, central heating is coming largely into use in public buildings, it is most necessary that it should be recognised that the true function of ventilation is to keep up the adequate loss of body heat and relieve the cutaneous sensations from monotony; that heating and ventilating systems must be kept separate and planned on the lines so well sketched out by Mr. Kimball; that architecture must be subservient to the demand for proper space, lighting, heating, and ventilation; and that intelligent, trained men (or women) are required to look after the systems of heating and ventilation, and work these so as adequately to cool, and avoid the monotony of, the occupants. L. H.
PROBLEMS OF MINE VALUATION.
THE State of Wisconsin has published a very interesting Bulletin under the title of "A Study of Methods of Mine Valuation and Assessment, with special reference to the Zinc Mines of South-western Wisconsin," by W. L. Uglow, which is well worthy of the attention of anyone concerned in the knotty problems of mine valuation. The Wisconsin Legislature has decided to impose a tax upon mineral property, which is to be valued on the same principle as is applied to all real estate in that State, namely, the 'full value which could ordinarily be obtained therefor at private sale"; this phrase is not quite clear, but apparently means the value that could be realised under a sale from a willing seller to a willing purchaser. The difficulties that beset such a valuation in all circumstances are increased by the fact that in the majority of cases these mines are leased by the mining companies from the royalty owners, whilst the necessary capital is often borrowed from banks, so that three separate parties are interested in the mining property. It is interesting to note that the royalty payable to the owner varies from 5 to 15 per cent., averaging 10 per cent. on the gross receipts, the owner paying as a rule the real property tax upon the increased value of his land due to the fact that it is mineral-bearing, as well as an income tax upon the royalty that he receives.
Two points of general interest are to be found in the treatment of the royalty and of the interest upon the capital borrowed for working the mine, the author holding that neither of these can equitably be considered as an addition to working costs, but that both must be looked upon as coming wholly out of profits. Probably few will question this treatment of interest, but the royalty question appears much more open to discussion. Unfortunately most of the other points dealt with have only a local rather than a general interest, because the ore-bodies are all shallow, and the life of mines in this district is consequently only a short one, so that the methods of valuation here employed have only a limited application.
A number of methods of valuation are discussed in detail, namely: (1) The Finlay ad valorem method, which calculates the value of a mine as the present value of the average annual profits that may be expected to be obtained during the life of that mine, setting asidė an annual sinking fund instalment, the total amount of all of which instalments, at the end of the estimated life of the mine, will amount to the present value. (2) The Arizona method, which takes the value of a mine for the purposes of taxation as represented by one-eighth of the gross production of the previous year, four times the net profit of the previous year and the value of the improvements. (3) The Colorado method, which takes the value as one-fourth of the value of the gross output of the preceding year, unless the net exceeds one-fourth of the gross, in which case the net output is taken to be the assessable value, except in the case of mines of the precious metals, where the value is taken to be half the value of the gross output plus the entire net output. (4) The Equated Income method, which takes the value as the present value of all future profits, these latter being taken as equal to the actual profits of the preceding year, excluding royalty, interest, and amortisation charges, assuming the same average length of life for all the mines of a district.
The author compares all these methods, and appears to decide in favour of the last-named. He is quite alive to the difficulties of the problem, and is also careful to point out the fact, too generally overlooked in the taxation of mines, that mining property is a wasting asset, and on this account ought not to be taxed on the same basis as ordinary real estate.
RECENT PROGRESS IN PYROMETRY.1
DURING the past five years, which is the period intended to be covered by the present paper, considerable advances have been made in the production of instruments for the measurement of high temperatures. Much valuable work in this direction has been carried out at the National Physical Laboratory in this country, and at the United States Bureau of Standards; and the manufacturers of pyrometers, chiefly in Britain and America, have introduced many new instruments for scientific and industrial purposes. Owing to the claims of atomic and molecular physics, the subject of pyrometry has not received that attention from physicists in general to which it is entitled by its industrial importance. The progress made in various directions may conveniently be considered under separate headings.
Standards of Temperature.
Various investigations of fixed points have confirmed, or at the most slightly modified, previously accepted figures. Up to the highest reading obtainable on the gas scale (1550° C.), the standards now in use appear to be well established; and beyond this the melting-point of platinum, as deduced by several different methods, is now accepted as 1755° C. This is a useful fixed point for the calibration of high-reading pyrometers.
The United States Bureau of Standards now issues materials of certified fixed points for the calibration of pyrometers. This procedure might with advantage be followed by our own National Physical Laboratory, so as to enable the indications of pyrometers to be checked from time to time by the user. If accompanied by instructions for use, a correct result would be ensured, and the danger of error resulting from the employment of materials of doubtfulˇ purity eliminated.
One of the chief features of recent years has been the extension of the use of base-metal junctions in place of platinum and platinum alloys. Suitable basemetal couples develop a relatively high E.M.F., and enable a strong and cheap indicator to be used in place of the sensitive instrument required for couples of the platinum series. Most makers now furnish iron-constantan pyrometers, which may be used up to 900° C. R. W. Paul employs two iron-nickel alloys of different composition which will register 1000° C.; whilst the Foster Instrument Company use two nickel-chromium alloys, capable of reading to 1200° C. in continuous use, and to 1300° C. for occasional observations. For temperatures below 700° C. copper-constantan junctions are much used, as, for example, in pyrometers for superheated steam. Various other base-metal couples are in use.
The trustworthiness of base-metal junctions has been called into question by Kowalke (Transactions of the American Electrochemical Society, vols. xxiv. (1913) and xxvi. (1914)). Trials made on junctions of this type, as supplied by American makers, showed changes in calibration on heating for twenty-four hours ranging from 20° C. to 130° C., the higher figure being obtained at temperatures of 1000° C. The results showed the necessity of "ageing" the junctions by prolonged heating at the maximum temperature prior to calibration. Experience with Britishmade junctions shows that large errors of this nature do not occur, owing to a careful choice of materials and suitable treatment before calibrating.
1 Abstract of a paper read before the Royal Society of Arts on May 12 by Mr. Chas. R. Darling.
C. C. Bidwell (Physical Review, June, 1914) has shown that a junction of carbon and graphite may be used to read temperatures as high as 2000 C. Previous heating to this temperature is necessary before calibrating in order to expel volatile matter, This junction gives promise of a valuable extension of the range of thermo-electric pyrometers, and may find industrial applications.
Materials for protecting junctions from the corrosive action of furnace gases have been added to by the introduction of alundum (oxide of aluminium), which melts at 2050° C.; a material known as "silit," which has a carborundum basis; and "silfrax," a substance resembling carborundum. All these materials are brittle, and therefore will not stand rough usage. As a protection for junctions used to read the temperature of molten brass or bronze, a tube of molybdenum has proved successful, as this metal is a good conductor of heat and is not acted on by the molten alloy.
Indicators for thermo-electric pyrometers have been improved in details by the various makers, resulting in greater trustworthiness. A new departure in commercial indicators has been made by the Leeds and Northrup Company of Philadelphia, who have adopted the potentiometer principle, formerly used only for accurate laboratory work. The connections are shown in Fig. 1, where B is a 2-volt accumulator, R, an
adjustable resistance, R, a fixed resistance, DE a uniform stretched wire, Ŝ a standard cell, G a sensitive galvanometer, and A a switch. In order to maintain a steady fall of potential along DE, R, is adjusted so that on switching the standard cell into the circuit of the galvanometer, no deflection is observed. As the voltage of B falls off with use, this adjustment must be made from time to time. pyrometer is connected to one terminal of the galvanometer and to a sliding contact F, which, in taking a reading, is moved along DE until no deflection is observed on G. From the known relation between temperature and E.M.F. for the junction used, DE may be graduated so as to read temperatures directly. The advantages of this arrangement are greater sensitiveness and independence of the resistance of the pyrometer and leads; the drawback, from an industrial standpoint, is that the readings are not automatic. Several methods for adapting an indicator for special ranges have been devised. In one form, due to the Cambridge Scientific Instrument Company, the indicator takes the place of the galvanometer G in Fig. 1, and by fixing the slider F at a given position-representing, say, 500°-the pointer of the indicator is prevented from moving until this temperature has been reached by the junction. The zero of the indicator is thus made to represent 500°, and
by keeping F fixed, the whole of the scale may be utilised for reading from 500° upwards. A more open scale, with correspondingly closer readings, may thus be obtained; and by suitable adjustment of F the range covered may be varied as required. The same firm has also introduced a mechanical device for achieving the same object. In this a suspendedcoil indicator is used, and by turning a torsion head a twist is imparted to the suspension, so that the pointer does not move over the scale until the temperature of the junction has reached an assigned figure. In the multi-range instrument made by R. W. Paul, the indicator takes the place of the galvanometer in a Wheatstone bridge circuit, the pyrometer being in series with the indicator. By throwing the bridge slightly out of balance, a current may be made to pass through the indicator in an opposite direction to that produced by the heated junction; and only when this opposing current is overcome will the pointer begin to move over the scale. Resistances are provided which, when inserted in the arms of the bridge, disturb the balance so as to hold up the indicator until an assigned temperature-say 600°-is
No special change is to be recorded in connection with this class of instrument. The recent work of Sir William Crookes has shown that platinum is distinctly volatile above 1000° C., and this explains satisfactorily why resistance pyrometers were found to change their calibration when used above this temperature. For work at low or moderate temperatures the resistance pyrometer is much used on account of its superior accuracy, which, however, is only operative when the temperature to be measured is subject to precise control. It is now customary to employ thermo-electric pyrometers for the general run of metallurgical work, and to use a resistance pyrometer for very exact work, and for a workshop standard within its upper limit--1000° C.
The researches of Northrup on the resistance of molten metals (Journal of the Franklin Institute,
FIG. 2.-Pyrometer for superheated steam.
reached, when the whole scale becomes available for the selected range. By using two mangani: and two copper resistances in the arms of the bridge, automatic correction is made for changes in the temperature of the cold junction. Thus, if the bridge were balanced at 20°, a current would flow through the indicator at any other temperature, as copper changes its resistance on heating or cooling, whilst manganin does not. The resistances are so chosen that the movement of the pointer caused by this current represents the increase or decrease in the temperature of the cold junction, and thus any errors due to this cause are eliminated.
The regulation of the temperature of superheated steam on locomotives has given rise to the problem of producing an indicator practically unaffected by vibrations. This has been solved in various ways by different makers, and satisfactory_instruments are now procurable for this purpose. Fig. 2 shows a pyrometer of this type made by Messrs. Siemens. Altogether, great progress has been made in this branch of pyrometry, particularly in the direction of greater accuracy.
FIG. 3.-Foster's radiation pyrometer for molten metals.
January and March, 1914) suggest a possible extension of the range of resistance pyrometers by the use of a liquid element. Melted copper, for example, shows a uniform increase in resistance up to 1400° C., and this fact might be utilised in measuring temperatures if a suitable appliance were forthcoming. The decrease in the resistance of pyro-conductors with rise in temperature may possibly be utilised for resistance pyrometers. Alundum, for example, has a resistance of 6100 ohms per cm. cube at 1100° C., which falls to 190 ohms at 1600° C.--an average decrease of nearly 12 ohms per degree, which could easily be detected on a coarse instrument.
A distinct improvement in these instruments has been the introduction of pivoted indicators in place of the suspended-coil type formerly in use. This has
been made possible by the use of more sensitive junctions to receive the radiations. The Féry pyrometer has been modified by Whipple for determining the temperature of molten metals by mounting the pyrometer at the open end of a fireclay tube, so as to be permanently focussed on the closed end which is immersed in the metal. Foster uses a similar plan in connection with his fixed-focus pyrometer (Fig. 3) the end being closed by a salamander tube which is dipped into the metal. The telescope is pierced by a tube open at both ends, through which air may be
FIG. 4.-Paul's radiation pyrometer.
forced with the view of preventing fumes from reaching and corroding the mirror in the event of breakage of the immersion tube. Mr. R. W. Paul has introduced a radiation pyrometer in which the rays are received in a tube containing a polished cone, a junction at the apex receiving the radiations. This type of instrument, originally due to Thwing, gives the same reading at all distances within a given limit, and therefore requires no focussing. A unipivot galvanometer is used as indicator (Fig. 4). A high
Laboratory of the Rudge-Whitworth Company, cells containing a liquid of the correct colour and density are used, so that the luminous rays from the heated object are extinguished at a given temperature. The instrument consists of an eye-shade (Fig. 5) in which four cells are mounted, two of which are capable c: extinguishing the light at the working temperature, whilst the other two correspond to a slightly higher temperature. Either pair may be brought before the eyes by moving a slide, and when the heated object is just visible in the lower pair, but invisible in the higher, the correct temperature has been attained. In another form a single cell is used, in which the depth of liquid may be adjusted until extinction occurs, and the temperature read off on a scale graduated in temperatures according to the depth.
Another form of extinction pyrometer has been designed by Alder and Cochrane (Patent No. 27,633, 1913), shown in Fig. 6. This consists of a small telescope, the rays through which are intercepted by a wedge of dark-coloured glass, which is moved until the sighted object just ceases to be visible. Temperatures are read from a scale which moves with the prism, and the range may be increased by inserting a piece of tinted glass in the eyepiece, and reading from a second scale prepared with the glass in position.
FIG. 6.-Alder and Cochrane's extinction pyro
An important paper, embodying results which suggest a new type of optical pyrometer, was read by Paterson and Dudding before the Physical Society in March, 1915. It was shown that the temperature of many metallic substances could be measured by matching the colour_against that of a black body until identical. A Lummer-Brodhun photometer was used, and the temperature of the black body varied until equality of hue was obtained. Filament lamps, with an ammeter and rheostat in the circuit, were thus matched, and became secondary standards, a given current corresponding to a known temperature. An optical pyrometer on these lines appears quite feasible.
Lovibond has suggested an optical pyrometer in which a standard source of light is brought to equality of tint with the heated object by interposing tintometer glasses.
FIG. 7.-Foster's recorder.
The value of temperature records is now so much recognised in the industries that all makers of pyro
meters have given special attention to recorders, and have devised arrangements for registering the readings of several pyrometers simultaneously. For resist. ance pyrometers the well-known recorder of Callendar is much used, and the modern form of Roberts-Austen photographic recorder, made by the White Instrument Company, is of great service in accurate work with thermo-electric instruments. Of the more recent types Foster's recorder (Fig. 7) possesses an indicator pivoted horizontally, the pointer being vertical and
Instrument Company has been improved in many details. One of the latest forms is shown in Fig. 8, in which two indicators are made to record on a single chart wound on a long drum rotated by clockwork. The presser-bar in this case pushes the pointer on to an inked thread, which touches and leaves an ink-dot on the graduated chart. By having two threads, coloured with different inks, and mechanism which brings each thread in turn beneath the pointer, four simultaneous records may be taken,
the pyrometers concerned being automatically switched on to the indicators in correct sequence.
A recorder in which the mechanism is driven by a motor has been introduced by the Leeds and Northrup Company, and is much used in the United States. R. W. Paul has also adopted the motor-drive in his new recorder (Fig. 9), which also embodies other novel features. The chart is made in the form of a continuous roll, lasting for 1000 hours, a large part of which is open to inspection through a window extension. The pointer is pressed periodically on to an inked ribbon, which
FIG. 9.-Paul's recorder.
moving across a circular chart rotating about its centre in twenty-four hours. At short intervals a presser-bar is urged against the pointer, at the end of which is placed a capillary tube containing an inked wick. A mark is thus made on the chart corresponding to the position of the pointer; and as the lines radiating from the centre are divided into temperatures, a complete record, visible in its entirety, is made.
The thread recorder of the Cambridge Scientific
FIG. 8.-Double thread recorder.
touches the chart at a place where the latter is passing over a knife-edge, and hence a dot is produced. Change-gear for altering the speed may be inserted when desired, and duplicate records are secured by means of a second ribbon, of different colour, which may be made to alternate in position with the first ribbon beneath the pointer. When a resistance pyrometer is in use, a Harris indicator replaces the galvanometer, a new chart, divided to suit this indicator, being inserted.