Construction of Platinum Thermometers. 107 



tions of an ohm in the manner now to be described, the appa- 

 ratus can at the same time be arranged so that the variable 

 resistance of the leads is eliminated. The reading of the 

 bridge-wire scale when the balance is found is then independ- 

 ent of the length of stem immersed, and gives the temperature 

 of the thermometer-coil without the necessity of observing 

 galvanometer-throws or of measuring the resistance of the 

 leads. 



The annexed figure represents, somewhat diagrammatically, 

 the arrangement of the apparatus. 



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J 



H 



A B, BC are equal resistances forming the arms of the 

 balance. The battery is connected at A and 0, and one 

 terminal of the galvanometer Gr at B. D E represents a set 

 of resistance-coils, which together with A B and B C may be 

 supplied by an ordinary post-office box. F K represents a 

 straight bridge-wire with a divided scale attached. The other 

 terminal of the galvanometer is connected to the contact- 

 piece H, which slides along this wire. The leads AM, K N, 

 from the pyrometer-coil P, are connected to A and K ; and the 

 compensating leads C L, L D, the resistance of which is equal 

 to A M, K N, are connected to C and D. These four leads 

 may be of any convenient length ; they are symmetrically 

 arranged so that corresponding parts are always at the same 

 temperature. When the balance is found by unplugging 

 suitable resistances in the arm D E and sliding the contact- 

 piece H, it is plain that, since the resistances A B, B C are 

 equal, the resistance of the pyrometer and its leads, plus that 

 of the length H K of the bridge- wire, will be equal to the 

 resistance of the remaining portion FH of the bridge-wire 

 together with that of the coils DE and the compensator CLD. 

 Thus the changes of resistance of the pyrometer-leads A M, 

 K N are compensated by the equal changes in the leads C L, 

 L D ; and the resistance of the pyrometer-coil itself is directly 

 given by the sum of the coils 1) E and the reading of the 

 bridge-wire. 



It is convenient to graduate the bridge-wire scale so that 

 100 or 1000 divisions are equivalent to the unit coil in the 

 arm D E. It is also convenient to adjust the resistance of the 



