66 Wright and Downs — Spectrum of the 



Art. VIII. — The Induced Alternating Current Discharge 

 studied with Deference to its Spectrum and especially the 

 Ultra- Violet Spectrum ; by A. W. Wright and E. S. 

 Downs. 



In some experiments made in this laboratory a few years 

 ago, in photographing the spectrum of an induced alternating 

 current discharge, produced under special conditions between 

 copper terminals, a plate was obtained which contained a very 

 great number of lines. The primary purpose of this investi- 

 gation was to locate these lines and if possible discover their 

 origin, and secondly to study this peculiar form of discharge 

 when produced on a very large scale. Spottiswoode* was 

 among the first to give an account of this mode of exciting 

 an induction coil by the direct application of an alternating 

 machine, without the intervention of a contact breaker or the 

 use of a condenser. In the Proceedings of the Koyal Society f he 

 points out some peculiarities of the discharge which he noted. 



In our experiments the induction coil was used without a 

 contact breaker or condenser. It had four hundred and twelve 

 turns of wire about 3 mm in diameter for its primary, and a 

 secondary of about 25*5 cm in length, consisting of about fifty 

 thousand turns of wire 0'3 mm in diameter. In the core of the 

 primary, which was 35'5 cm long and 3"5 cm in diameter, there 

 were nearly a thousand soft iron wires carefully annealed and 

 insulated with shellac. As a result of having such large 

 wire for its secondary, the current was very large, and the 

 potential as determined by the length of the spark was in the 

 neighborhood of 120,000 volts. The current was supplied to 

 the coil by a Siemens alternating current dynamo of about five 

 horse power which was driven by a gas engine, and whose 

 magnets were excited by a small dynamo using approximately 

 three horse power. 



With the terminals arranged horizontally the discharge was 

 exceedingly intense and vigorous, being accompanied with a 

 loud singing noise. It consisted of a light nebulous flame of 

 a whitish color, with a slight tinge of yellow or green, rising 

 gradually from the terminals and meeting at the center, where 

 the color changes to a reddish. The flame appeared to the eye 

 to be continuous, but, when observed in a rotating mirror or 

 photographed on a swiftly moving plate, the alternate dis- 

 charges were plainly discriminated. About and close to the 

 terminals the bluish-purple flame, caused by the nitrogen of 

 the air was plainly seen. The spark would leap across an 

 interval of about 3*5 cm , and when the discharge was once 

 started, the terminals could be drawn apart for as much as 20 cm 



* Phil. Mag., Nov. 1879. f Vol. xxx, p. 173. 



