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VII. Experimental Proof that the Electric Spark is an Electro- 

 motor. By E. Edlund*. 



IN former papers f I have proved that by the mechanical work 

 which the voltaic current exerts in disintegrating the poles in 

 producing the electric light, an electromotive force results which 

 sends a current in a direction opposite to that of the principal 

 current. As, in an electric discharge, when sparks are formed in 

 the air there is also a disintegration of the polar surfaces, the idea 

 readily suggested itself that the electric spark, like the voltaic 

 arc, might possibly be an electromotor. To investigate this point 

 more minutely, I have, in common with Dr. Lemstrom of Helsing- 

 fors, made the following experiments. 



A Holtz's machine, made by Ruhmkorff of Paris, was used in 

 the investigation. The rotating glass disk had a diameter of 55 

 centims., and each of the jars belonging to the apparatus had a 

 coated surface of 42 square centims. The copper galvanometer 

 wire was 1 millim. in diameter, and was surrounded by a layer 

 of gutta percha 2 millims. in diameter. The entire thickness of 

 the wire, the insulating layer included, amounted to 5 millims. 

 This wire was coiled in forty turns round a wooden frame. The 

 aperture inside the frame, in which the needle was suspended by 

 a cocoon-thread, was 5 centims. in length and 3 centims. high. 

 To protect the needle from draughts, the instrument was enclosed 

 in a glass jar. 



2. If the voltaic current which, it is assumed, is produced in 

 the electric spark, is to be investigated by the aid of the galva- 

 nometer, the spark must be connected with the galvanometer by 

 a closed circuit. This necessary condition was satisfied, after 

 many vain attempts, in the following manner : — 



In the following figure, AB represents the rotating disk, and ab 

 the two combs. An insulated copper wire a c is directly con- 

 nected with a, while the insulated wire de terminates in a brass 

 knob d in the neighbourhood of b. From c and e insulated wires 

 pass to the knobs / and g. Two other conducting-wires lead 

 from the points c and e to the galvanometer G. At m a rheostat 

 is introduced, which consists of an insulated thin German-silver 

 wire. If the machine is charged and the disk A B put in rota- 

 tion 5 and if moreover the resistance at m and the distance between 

 the knobs fg and b d is suitably adjusted, sparks pass simulta- 



* Communicated by the Author, as a separate copy of a paper pub- 

 lished in Poggendorff's Annalen, July 1868, having been read before the 

 Swedish Royal Academy of Sciences on the 13th of May 1868. 



t Poggendorff's Annalen, vol. cxxxi. p. 586, and vol. cxxxiv. p. 250. 

 Phil. Mag. vol. xxxv. pp. 103 & 441. 



