Potts — Rowland's New Method. 97 



employed. Both were directly connected to small electric 

 motors. Both had armatures of the pancake type. The one 

 had four coils in the armature and four poles and thus pro- 

 duced a current of two complete periods for each revolution of 

 the armature. The other was larger but of similar construc- 

 tion, having six coils in the armature and six poles, and gave 

 three complete periods for one revolution. If the load on the 

 dynamo was not changed, these dynamos would run at a very 

 nearly constant speed. With the second dynamo, the number 

 of complete periods per second could be varied from 6 to 70. 

 However, at the lower speed the electro-dynamometer was 

 difficult to balance, since the hanging coil would vibrate with 

 the current and blurr the image of the scale. The voltage 

 furnished could be controlled very well by changing the 

 strength of the field. In any one series of observations the 

 same dynamo was used, as the results using different dynamos 

 would not be comparable, on account of the different har- 

 monics introduced. The small dynamos which were almost 

 exclusively used gave, however, very good sine curves. 



Resistances. — The high resistances and those which were 

 required to carry very small currents were made of fine ger- 

 man-silver wire wound on thin sheets of fiber. The self-induc- 

 tion and electrostatic action of these was practically zero. The 

 lower resistances, and those required to carry larger currents 

 were made of a special resistance wire, which had a very slight 

 negative temperature coefficient, and would bear considerable 

 heating with a very small change of resistance. This wire was 

 wound on slates. -Each slate contained sufficient number 30 

 wire to have nearly 2000 ohms resistance. These were con- 

 veniently subdivided for adjustment. For the final adjust- 

 ment an ordinary resistance box was used, but never more than 

 fifty ohms were used in this box, and then the total resistance of 

 that arm was at least 1000 ohms. 



Self -inductance. — Two coils were used. 



A. External diameter 35-16 cm ; internal diameter 23-8 0m , 

 3700 turns No. 20 B. and S. Self-inductance 5-30 henrys. 

 Resistance 188 ohms. 



C. Same dimensions as A except depth. Self-inductance 

 1*30 henrys, 1747 turns No. 22 B. and S., single cotton cov- 

 ered copper wire. Resistance about 78 ohms. 



Condensers. — 2 and 3. Paper condensers made by Marshall 

 of 2 and 3 micro-farads capacity. 



Willy oung. — 8 micro-farad wax condenser, made by Will- 

 young & Co. and divided into sections of one micro-farad each. 



Mica condensers. — -| M. F. standard condenser made by 

 Elliot Bros. 



