1883.] On the Electrical Resistance of Carbon Contacts. 11 



easily overcome by a small electromotive force than by a high one. 

 Though the experiment as tlms described is generally successful, it 

 occasionally happens that the single cell fails equally with the larger 

 battery to overcome the resistance. 



For a systematic investigation of the last-mentioned group of 

 phenomena the apparatus was arranged as shown in fig. 6. 



It is essentially a Wheatstone's bridge arrangement, the propor- 

 tional coils used being 1 and 1,000 ohms. A tangent galvano- 

 meter of *9 ohm resistance is inserted in the same arm as the carbon 

 balance, and a box of resistance coils R is placed between the battery 

 (which consists of eight or ten Leclanche cells) and the bridge. The 

 arrangement admits of the measurement of the resistance of the 

 carbon when traversed by strong currents, the strength of which can 

 be regulated by the resistance box R, and measured by the tangent 

 galvanometer Gr. A given weight being in the scale-pan, known 

 currents of gradually increasing strength were passed through the 

 carbon. The resistance was measured, 1st, while the known current 

 was passing ; 2nd, with a very small current obtained by unplugging 

 10,000 ohms in the resistance box. As the currents increased in 

 strength, the resistance measured with the very small current gra- 

 dually diminished until a point was reached — and the lighter the 

 weight the sooner this was the case — at which the resistance sud- 

 denly became infinity. The measurements are given in Table IX, the 

 currents varying from '01 to '7 ampere, and the pressures from 

 -05 grm. to 5 grms. 



