88 



Ml,'. K. K. SMITH ON THE CONSTRUCTION OF 



which 1690 is expressed is therefore assumed to be equal to 10 9 C.G.S. units of 

 resistance (see pp. 114 and 115). 



By introducing P' into the circuit (the diagram shows it in a displaced position), 

 the value of QR/S may be obtained. 



When P' is displaced, connections can be effected to the mercury standard. The 

 leads of the latter were secured as already described to insulated terminals on the 

 cover of the ice box. From three of these terminals ^ -inch copper leads passed to 



. c - 



V75 



00/J//W C 



S TAt OARL 



UR\' 



O-50 



Q'2.5 



\ 



O'OO 



60 



I HOUR 



a 



JOO 



J2O J4O 



a HOURS 



160 



J80 



3 HOURS. 



Fig. 11. 



mercury cups in thick copper blocks, which latter formed part of the bridge. To the 

 fourth terminal was secured one of the battery leads, and the connections thus rendered 

 complete. 



In practice, the tapping current employed was about 0'2 ampere, and with this a 

 sensitivity corresponding to the evaluation of the resistance to O'OOOl per cent, was 

 obtained. The galvanometer was one constructed at the laboratory with consequent 

 pole magnets for the astatic system, as designed by Professor A. BROCA, of the Ecole 

 de Me"decine, Paris. Its resistance was 500 ohms. The deflections were read 

 telescopically at a distance of 4 metres, the scale divisions being transparent on a 

 dark ground, with sunlight as an illuminant. 



