50 



NATURE 



\May 16,1872 



one comprising the spiral of Ihin wire, and the other re- 

 turning immediately in front of the same, but traversing 

 in its stead a comparison coil of constant resistance. 

 The measuring instrument may consist of a differential gal- 

 vanometer as before, if to the constant resistance a variable 

 resistance is added. If the pyrometer coil were to be put 

 into a vessel containing snow and water, tlie balance of 

 resistance between the two battery circuits would be 

 obtained without adding variable resistance to the coil of 

 constant resistance, and the needle of the differential 

 galvanometer would remain at zero when the current is 

 established. But on exposing the pyrometer to an elevated 

 temperature, the resistance of its platinum coil would be 

 increased, and resistance to the same amount would liave 

 to be added to the constant resistance of the measuring 

 instrument, in order to re-establish the electrical balance. 

 This additional resistance would be the measure of the 

 increase of temperature, if only the ratio in which platinum 

 wire increases in electrical resistance with temperature is 

 once for all established. This is a question which 1 shall 

 revert to after having completed the description of the 

 pyrometric instrument. 



Although I have explained that bymeans of a differential 

 galvanometer and a variable resistance (constituting in 

 effect a Whcatstone bridge arrangement) the increasing 

 resistance of the platinum spiral may be measured, it was 

 found that the use of a delicate galvanometer is attended 

 with considerable practical difficulty in iron-works and 

 other rough places where it is important to measure 

 elevated temperatures, or on board ship for measuring 

 deep-sea temperatures. 1 was tlierefore induced to seek 

 the same result by the conception of an instrument which 

 is independent in its action from tremulous motion, 

 or from magnetic disturbance caused by moving masses 

 of iron, and which requires no careful adjustment or 

 special skill on the part of the operator. This instrument 

 is represented by Fig. 3 on page 4S, and may be termed 

 a chemical resistance measurer or " differential volta- 

 meter." The immortal Faraday has proved that the de- 

 composition cf water in a voltameter expressed by the 

 volumes of gases V, is proportionate in the unit of time 

 to the intensity I of the decomposing current, or that 

 V 



I = 



T" 



According to Ohm's general law, the intensity I is 

 governed by the electro-motive force E, and inversely by 

 the resistance R, or it is 



E 



I = 



It is therefore 



orV = 



R 



or the volume V would give a correct measure of the elec- 

 trical resistance R, if only the electro-motive force E and 

 time T were known and constant quantities. But the electro- 

 motive force of a battery is very variable ; it is influenced 

 by polarisation of the electrodes, by temperature, and by the 

 strength and purity of the acid employed. The volume of 

 gases obtained is influenced, moreover, by the atmospheric 

 pressure, and it is extremely difticult to make time obser- 

 vations correctly. It occurred to me, however, that these 

 uncertain elements might be entirely eliminated in com- 

 bining two similar voltameters in such a manner that the 

 current of the same battery was divided between the two, 

 the one branch comprising the unknown resistance to be 

 measured, and the other a known and constant resistance. 

 The volume of gas V produced in this second voltameter, 

 having a resistance R' in circuit, would be expressed by 



V = 



ET 

 R' ' 



V : V 



and we should have the proportion of 

 E T . E T. 

 "R ■ R'' 



or E and T, being the same in both cases, may be struck 

 out, and the expression will assume the simple form 



V : V = R : R. 

 The constant resistance R of the one circuit being known, 

 it follows that the unknown resistance R' is expressed by 



- ; that is to say, by a constant multiplied by the propor- 

 tion of gas produced in the two voltameters irrespective 

 of time, or strength of battery, or temperature, or the 

 state of the barometer. 



The resistance R and R' are composed each of two re- 

 sistances, namely, that Of the principal coils, which we 

 may term R or R', and of the voltameter and leading 

 wires, which is the same in both cases, and may be ex- 

 pressed by t'. The expression should therefore be written 

 as follows : — 



V : V = R' 4-^1 : R + y, 

 R' being the unknown quantity. 



The mechanical arrangement of the instrument will be 

 understood from the diagram. Fig. 3 ; and the whole 

 arrangement of the pyrometer, with its leading wire and 

 resistance measurer, from the general view given in Fig. 5. 

 The voltametric resistance measurer consists of two cali- 

 brated vertical tubes of glass of about 3 millimetres 

 diameter, which are tixed upon a scale showing arbitrary 

 but equal divisions. The upper ends of the tubes are 

 closed by small cushions of india-rubber pressed down 

 upon the openings by means of weighted levers, whereas 

 the lower portions of the tubes are widened out and closed 

 by plugs of wood, through which the electrodes in the 

 form of pointed platinum wires penetrate to the depth of 

 about 25 millimetres into the widened portions of the 

 tubes. By a side branch the widened portion of 

 each vertical tube communicates by means of an india- 

 rubber connecting pipe to a little glass I'eservoir con- 

 taining acidulated water, and supported in a vertical 

 slide. In raising the weighted cushions closing the 

 upper ends of the vertical tubes, and in adjusting the 

 position of the small reservoirs, the acidulated water will 

 rise in both tubes to the zero line of the scale. In turning 

 a button in front of the tubes the battery current is passed 

 through both pairs of electrodes, the one circuit com- 

 prising the permanent resistance R and the leading wires 

 up to the pyrometer, and the other the leading wires and 

 the pyrometer coil. If the resistance of the pyrometer 

 coil should be equal to the permanent resistance R, then 

 R' -\- y wdl be equal to R +_)', and therefore V = V', but 

 as the resistances differ, so will the volumes. Necessary 

 conditions are, that both reservoirs are filled with the 

 same standard solution of pure water with about 10 per 

 cent, of sulphuric acid, that all the electrodes are of the 

 same form and size, and that their polaiity is reversed 

 frequently during the progress of each observation, in 

 oroer to avoid unequal polarisation. With these pre- 

 cautions, which involve no particular skill or knowledge of 

 electrical observation en the part of the operator, very 

 accurate results are obtained ; but in order not to incur con- 

 siderable errorof observation, it isadvisable nottocontinue 

 the current, reversing the same, say twice, until at least 

 forty divisions of gascs are produced in the least activated 

 tube, which operation will occupy from two to three 

 minutes, if a battery of from four to six Daniel elements 

 is employed. The volumes V and V being noted, after 

 having allowed half a minute for the gases to collect after 

 the curient has ceased, the weighted cushions upon the 

 tubes are raised in order to allow the gases to escape, 

 when the water levels will immediately icturn to their 

 zero position, to make ready for another observation. By 

 inserting the observed values for V and V into the ex- 



