560 Prof. H. L. Callendar on Electrical 



the same balancing coil, provided that allowance is made in 

 this way for the small differences between them. 



Testing for Defects. 



9. To test the thermometer and compensator leads for 

 defective insulation disconnect one of the thermometer leads, 

 say C 2 , from the battery, and the opposite compensator lead, 

 say P l5 from the bridge-wire (or vice versa, C x from the 

 bridge-wire and P 2 from the battery). If, now, the galva- 

 nometer circuit is made, there should be no deflexion if the 

 insulation is perfect, since the bridge-wire and galvanometer 

 contact are effectively isolated from the rest of the circuit,, 

 unless there is defective insulation between the thermometer 

 and compensator. Before disconnecting the wires for this 

 test the galvanometer circuit should be broken, otherwise the 

 instrument will be violently agitated, and its zero may be 

 affected. 



To measure the heating effect of the working current on 

 the thermometer, place the thermometer in water at a steady 

 temperature, balance the bridge as already explained, and 

 adjust the galvanometer scale if this has not been done. 

 With the galvanometer balanced at zero, and the current 

 adjusted to its proper value, change the battery from one 

 storage cell to two cells of the same E.M.F. Since the 

 resistance of a storage cell is negligible compared with the 

 other resistances in the circuit, this change will have the 

 effect of doubling the current through the thermometer. 

 The rise of temperature of the thermometer due to the 

 passage of the current will be quadrupled, since it varies as 

 the square of the current. The change of resistance of the 

 thermometer due to changing from one cell to two, will be 

 three times the change of resistance due to the heating effect 

 of the current with one cell. The deflexion of the galvano- 

 meter for a given change of resistance will also be doubled, 

 since the current is doubled. The deflexion of the galvano- 

 meter produced by changing from one cell to two will 

 therefore be six times the heating effect with one cell 

 measured in degrees of temperature. Hence observe the 

 galvanometer deflexion in degrees of temperature and divide 

 by six to find the heating effect for the normal current with 

 one cell. If the heating effect so deduced does not exceed 

 a tenth of a degree, it will not introduce any appreciable 

 error, since it remains practically constant, so long as the 

 scale adjustment is the same. With a given thermometer 

 and galvanometer the heating effect varies as the square of 

 the number of scale divisions per degree. Thus if the 



