330 



Mr. W. P. Boynton on the 



The effective currents were measured by a form of hot-wire 

 ammeter or dynamometer due to Hertz *. The current 

 traversed a fine german-silver wire which held a small steel 

 wire in equilibrium against the torsion of a spring (fig. 4) . The 



Fig. 4. 

 WoocC 



/J<zr&? /fu-dder 



Steel Were 



heat due to the current expanded the wire and allowed the 

 steel wire to rotate under the influence of the spring. The 

 deflexions were read with mirror and scale. These instru- 

 ments were repeatedly calibrated, using a storage battery and 

 known resistances, or current from a step-down transformer 

 through a known non-inductive resistance, or by comparison 

 with various Weston ammeters. The results were gratifyingly 

 uniform. The sizes of wire used were numbers 30, 36, 40, with 

 carrying capacity varying from 2 to *5 amperes. The instru- 

 ments were very deadbeat, and particularly in the case of the 

 smaller wires came to the final readings very promptly and 

 returned to zero almost as promptly. 



The sensitive quadrant electrometer just described was 

 connected in parallel with the secondary capacity K 2 , and the 

 two dynamometers were inserted in convenient positions in 



?0 Generator 



, v Absolute Electrometer. 

 E 2 . Sensitive Electrometer. 



D lf D 2 . Dynamometers. 

 Kj, K 2 . Condensers. 



the primary and secondary circuits. After many trials of 

 different positions, the dynamometer for the primary circuit 

 was placed in the branch containing the spark-gap (fig. 5). 



The terminals of the secondary circuit of the large induction- 

 coil were permanently connected to the absolute electrometer, 

 as well as to the primary condenser of the oscillatory system. 

 * Zeitschr.fur Inst. iii. pp. 17-19 (1883) ; Ges. Werke, Bd. i. p. 227. 



