ing 



344 Mr. J. Trowbridge on the Oscillations of 



have obtained evidence that successive discharges of lightnin 

 follow the same path. Professor Lodge has protested, with 

 reason, against the conclusions drawn by the method of 

 " waggling " the head or a camera ; for the movement of the 

 head or the camera certainly requires the hundredth of a 

 second, while the discharge of lightning is over in less than 

 one hundred-thousandth of a second. 



The method of photographing electrical discharges by 

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

 studying the behaviour of air which is suddenly subjected 

 to the electric strain. I have, therefore, examined this be- 

 haviour with more powerful means than those employed by 

 previous observers ; and it may be well to recall here the fact 

 that in lightning discharges high electromotive force and 

 great quantity are frequently combined in a very short 

 interval of time. The modern alternating machine, there- 

 fore, and the device of the transformer enable one to study 

 the character of lightning more successfully than is possible 

 by means of an electrical machine ; for both the electromotive 

 force of a discharge and its quantity can be adjusted over a 

 wide range. In my study of this subject I employed an 

 alternating machine which gave three hundred to four hun- 

 dred alternations per second and a current of from fifteen to 

 twenty amperes ; and the photographic apparatus was the 

 same as that which was used in my investigation on the 

 damping of electrical oscillations on iron wires*. By means 

 of a step-up transformer and an oil-condenser, discharges of 

 high electromotive force and great quantity could be readily 

 obtained. The method of the excitation of a Kuhmkorff coil 

 or transformer by means of an alternating dynamo — due 

 originally to Spottiswoode — has placed in the hands of the ex- 

 perimenter, as I have said, powerful means of studying electric 

 discharges ; and by the device of an air-blast or other con- 

 trivance 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. Since 

 my object, however, was to study the photographs of sparks 

 having both great electromotive force and great quantity, I 

 limited myself to discharges of about two centimetres. Figs. 1 

 and 2 (PI. III.) are reproductions 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-thousandth 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 

 * Phil. Mag-. Dec. 1891. 



