OP ARTS AND SCIENCES. 119 



instant contact. This seems to us improbable, for a deep and clear- 

 cut groove was made in the soft type-metal. Great attention was 

 paid to the solid structure of this contact apparatus. It was entirely 

 separate from the support of the revolving parts, and was perfectly 

 steady. 



The other end of the armature shaft was lengthened into a cylin- 

 drical chronograph, similar to that described in the article already 

 cited, and its performance left nothing to be desired. A small 

 Kuhmkorf coil, excited by two storage cells, and interrupted by a 

 seconds pendulum, gave a record 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 car- 

 riasfe, which, ou beins 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 Topler 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 expe- 

 rience of the previous year, and worked for months without failure ; 

 and the taking of photographs of the oscillatory discharge by it be- 

 came a mere matter of routine. 



The following cases were tried : — 



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

 .087 cm.), the number of double oscillations visible on the negatives 

 averaged quite uniformly 9 or 9.5. 



(2.) When the wires were of German silver (diameter .061 cm.), 

 three oscillations were visible. 



(3.) But when an annealed iron wire (diameter .087 cm.) was 

 substituted, only the first return oscillation was distinctly visible, with 

 occasionally a trace of the first duplicate discharge. 



(4.) On substituting fine copper wire (diameter .027 cm.), five 

 complete oscillations were quite uniformly visible. 



(5.) Fine German silver wire (.029 cm.), nickel wire (.019 cm.),* 

 soft iron (.027 cm.), and piano steel wire (.027 cm.), 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 



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



