2 E. L. Nichols — Alternating Electric Arc. 



interrupter and condenser of which had been thrown out of cir- 

 cuit. The primary coil, as in ail instruments of that type, con- 

 sisted of a few turns of heavy wire, surrounding a core of iron 

 wires. When this coil was supplied with current from a small 

 alternating current dynamo, making 14000 reversals a minute, 

 and the terminals of the secondary coil were brought into posi- 

 tion, a discharge of considerable brilliancy took place between 

 them. To the unaided eye, the discharge appeared to be 

 perfectly continuous, but the fact that it was really of an 

 intermittent and periodic character, was indicated by the emis- 

 sion of a well defined musical note, which corresponded in 

 frequency with the period of alternation of the dynamo. 



The terminals of the secondary coil were subsequently con- 

 nected with a brass ball, about one centimeter in diameter, and 

 with a point consisting of a steel sewing needle. These were 

 mounted horizontally in well insulated bearings, the center of 

 the ball in line with the axis of the needle. The distance be- 

 tween the ball and the point of the needle was capable of ad- 

 justment by means of a micrometer screw. A mirror galva- 

 nometer of two thousand ohms, having in its outer circuit 

 about one hundred thousand ohms, was shunted around the ball 

 and point (in parallel circuit with the air space between them). 

 When the induction coil was put into operation, the ball and 

 point being too far apart to admit of a visible discharge, the" 

 galvanometer needle remained at zero, but when they were 

 brought within striking distance, a large and constant deflection 

 was produced. When the ball and point were interchanged, the 

 deflection was reversed, its direction always being that which 

 would have resulted from a current flowing from the ball to the 

 point. Under the influence of the discharge, which was intense- 

 ly luminous, the steel needle was fused at the point and rapidly 

 wasted by oxidation, so that it became necessary to find some 

 more refractory material.* It was finally supplanted by a 

 pointed platinum wire, which although rendered highly incan- 

 descent withstood the temperature of the arc much better than 

 steel had done. 



The following quotation will serve to indicate the conclu- 

 sions reached by the observers in the course of their preliminary 

 experiments with the platinum point : 



*The attempt to use carbon terminals, led to the following observation. " A 

 carbon pencil substituted for the point, gave the same effect, but upon putting the 

 carbon in place of the ball, it still acted as a 'point' If two carbons were used 

 the more pointed one acted as a ' point.' — It was observed that the end of the 

 needle was fused into a ball by the heat of the arc, and would then act as a ' ball ' 

 to the smaller particles of carbon projecting from the end of the pencil. — Two 

 brass balls brought together, caused a drifting of the galvanometer needle from 

 one side to the other, according, it is to be presumed, as the discharge changed 

 the nature of the two surfaces, so that minute points formed on one or the 

 other." (Archbold and Teeple ; Thesis, p. 3.) 



