Barker | 



L08 



[Nov. I... 



other two one had a constant of three ohms, and the other contained the 

 tasimeter which was adjusted by means of the screw to three ohms. When 

 thus balanced, if the strip of vulcanized rubber A (seen in Fig. 3), placed 

 between the fixed point 15 and the carbon button C, was exposed to heal 

 from any source, it expanded, placing pressure upon the carbon button, 

 decreasing in this way its resistance and destroying the balance ; thus allow- 

 ing a current to pass through the bridge wire containing the galvanometer, 

 tin- amount of this current of course being proportional to the expansion of 

 the rubber and to the strength of the battery. 



"The form of instrument here described was finished only two days 

 before leaving tor the west; hence, I was unable to test it. However, I 

 set it up upon my arrival at Rawlins, but found that it was a very difficull 

 matter to balance so delicate an instrument as a reflecting galvanometer 

 with one cell of battery, through such small resistances. In fact, I did not 

 succeed in balancing it at all in the usual way. Nor could it be balanced 

 in any way until I devised a. method which I may designate 'fractional 

 balancing,' when it became very easy to accomplish the result and also to 

 increase the effect by using two cells in place of a single one. This device 



Fiff.8. 



consisted of a. rheostat formed of two rows of pins. The rows were about 

 one-half an inch apart. A wire was connected from a pin on one row to a 

 pin on the other row and so on, so that the current had to pass through 

 the whole length of the wire, which was No. 24 gauge and four feet long. 

 This was used as a shunt around the galvanometer. A copper wire con- 

 necting all the pins of one row served to reduce the resistance to zero. 

 When the galvanometer was thus shunted, a very feeble current passed 

 h il. II He- spot of lighl was not at zero it was brought there by 

 either increasing or decreasing the pressure upon the vulcanite of the tasi- 

 meter by the adjusting nut. When thus brought to zero the copper wire 

 of the shunt rheostat was taken off of one pin, thus increasing the resist- 



