508 Mr. J. Trowbridge on Damping of 



of the speed of the mirror. The stylus which drew the spiral 

 turns on the barrel of the chronograph was drawn along 

 the barrel by means of a small heavily loaded carriage, 

 which, on being released at the moment the lever arm threw 

 the type-metal in contact with the cutting-tool, descended an 

 inclined plane of adjustable height. A small Toepler Holtz 

 machine charged a large Leyden jar, and it was found to work 

 admirably in all states of the weather. 



The apparatus which I have thus described was the result 

 of the experiments of the previous year and w r orked for months 

 without failure ; and the taking of photographs of the oscil- 

 lating discharge by it became a mere matter of routine. 



The following cases were tried : — 



(1) When the long parallel wires were of copper (diameter 

 '087 centim.), the number of double oscillations visible on the 

 negatives averaged quite uniformly 9 or 9*5. 



(2) Where the wires were of German silver (diameter 

 •061 centim.), 3 oscillations were visible. 



(3) But when an annealed iron wire (diameter "087 centim.) 

 was substituted, only the first return oscillation was distinctly 

 visible, with occasionally a trace of the first duplicate discharge. 



(4) On substituting tine copper wire (diameter '027 centim.), 

 5 complete oscillations were quite uniformly visible. 



(5) Fine German silver wire ('029 centim.), nickel wire 

 (*019 centim.)*, soft iron ("027 centim.), and piano steel 

 wire ('027 centim.), gave but faintly the first return discharge 

 after the pilot spark. 



The pilot sparks were in all cases strong. 



The single return discharge through the iron wire did not 

 admit of measurement sufficiently accurate to furnish any 

 basis for calculation of its self-induction. The time did not 

 apparently differ, if at all, by more than 14 or 15 per cent. 

 Some general reasoning based upon the number of oscillations 

 may be of interest. It must be acknowledged, however, that 

 this reasoning is open to criticism, although it affords the 

 most plausible explanation of the phenomenon. The phe- 

 nomenon itself is not a doubtful one. 



The time of a double oscillation for the large-sized copper 

 wire was '0000020 sec, for the small copper wire '0000021 

 sec. The others, as far as could be determined, did not differ 

 much from these values, and for this purpose either value is 

 sufficiently accurate. Denote by B/ the ohmic resistance of 

 the parallel wires to alternating currents of this periodicity, 

 by B the resistance to steady currents. 



* Obtained by the kindness of Joseph. Wharton, Esq., of Philadelphia. 



