196 J. Trowbridge — Oscillations of Lightning discharges 



head or camera certainly requires the hundredth of a second 

 while the discharge of lightning is over in less than one hun- 

 dred thousandth of a second. 



The method of the photography of electrical discharges by 

 means of a revolving mirror seems to be the best method of 

 studying the behavior of air suddenly subjected to the electric 

 strain. I have therefore examined this behavior with more 

 powerful means than those employed by previous observers ; 

 and it may be well to recall here the fact that usually in light- 

 ning discharges, high electromotive force and great quantity 

 are frequently combined in a very short interval of time. 

 The modern alternating machine and the device of the trans- 

 former enables one to study the character of lightning more 

 successfully than is possible by means of an electrical ma- 

 chine : for both the electromotive force of the discharge and 

 its quantity can be adjusted over a wide range. In my study 

 of this subject I employed an alternating machine giving from 

 three hundred to four hundred alternations per second, with 

 a current from fifteen to twenty amperes, and the photographic 

 apparatus was the same as that which was used in my investiga- 

 tion on the damping of electrical oscillations on iron wires.* 

 By means of a step-up transformer and an oil condenser, dis- 

 charges of high electromotive force and great quantity could 

 be readily obtained. The method of the excitation of a 

 Ruhmkorf coil or transformer by means of an alternating 

 dynamo, due originally to Spottiswode, has placed in the hands 

 of the experimenter, as I have said, powerful means of studying 

 electrical discharges. 



Since my object was to study the photographs of sparks 

 having both great electromotive force and great quantity I 

 limited myself to discharges of about two centimeters. By 

 the device of an air blast and other contrivances for obtaining 

 a quick break in the continuity of the electrical discharges, 

 Professor Elihu Thompson has shown how sparks of many 

 feet in length can be obtained. Figures 1 and 2 are repro- 

 ductions from untouched negatives not enlarged, and ten to 

 twelve oscillations can be counted on each photograph. The 

 interval between the oscillations is about one hundred thou- 

 sandth of a second ; and it will be noticed that the electrical 

 discharge follows exactly the same path in the air for three 

 hundred thousandths of a second. During this length of time 

 every sinuosity in the air path is exactly reproduced. I em- 

 ployed terminals of tin ; and on figure 1 it will be noticed that 

 a mass of melted and vaporized tin remained suspended in the 

 air for at least three hundred thousandths of a second before 

 it was dissipated in a comet-like tail. During the three hun- 



*Phil. Mag., Dec, 1891. 



