544 Prof. H. L. Callendar on Electrical 



smaller, R = 2*56 ohms, the best resistance for the galyano- 

 meter would be 4*5 ohms. This could be secured approxi- 

 mately by putting the two coils of the 16 ohm galvanometer 

 in parallel, but the advantage gained thereby would be only 

 10 per cent. If, on the other hand, the ratio coils had been 

 made equal to 25*6 ohms each, according to the rule given 

 by Maxwell and generally followed, there would have been 

 a loss of sensitiveness of 20 per cent, with the 25*6 ohm 

 thermometers, and 50 per cent, with the 2' 56 ohm thermo- 

 meters, which could not so well be neglected. 



Resistance of the Thermometer. 



5. For a platinum thermometer of resistance R at 0° C, 

 the change of resistance per 1° C. is approximately *004 R , 

 which must be substituted in equation (1) for the value of 

 dR or R — R', in order to find the deflexion of the galvano- 

 meter per degree change of temperature. Making this 

 substitution, and remembering that n = l, we find that, when 

 the resistance of the thermometer is changed, the deflexion 

 per degree, which is proportional to c\/G, varies as 



RCv / S/(2G+(l + m)R). 



The rise of temperature produced in the thermometer by 

 the measuring current C is directly proportional to C 2 R, and 

 inversely proportional to the rate of dissipation of heat per 

 degree rise of temperature of the thermometer above its 

 surroundings. The ,rate of dissipation of heat depends on 

 the form and surface of the thermometer, and on the con- 

 ditions of exposure. For similar thermometers of different 

 resistances, but of the same size, under similar conditions of 

 exposure, we must have G 2 R the same in order to secure the 

 same degree of accuracy as limited by the heating effect of 

 the current. The permissible current C will therefore vary 

 inversely as the square root of the resistance of the thermo- 

 meter. Making this substitution, we have 



c\/G varies as \/'GR/(2Q + (l + m)'R). 



We see from this result that, if it is possible to choose the 

 best resistance for the galvanometer, namely (l + m)R/2,. 

 and if m is the same for all, the sensitiveness and accuracy 

 of the thermometer, so far as the heating effect of the 

 current is concerned, will be independent of its resistance. 

 In accurate laboratory work, where a delicate galvanometer 

 is available, the heating effect of the current is so small as 

 to be relatively unimportant, and the resistance of the 

 thermometer is chosen chiefly with a view to minimise errors 



