Photography of the Static Discharge

Abstract

The accompanying photograph of the spark of a large static machine may possibly be of some interest to the readers of NATURE. The machine is a large Holtz, used in the electrical department of St. Bartholomew's Hospital. It consists of eight glass plates of twenty-nine inches in diameter, inclosed in a glass case. It is driven by a motor which is worked by the 100 volt alternating main which supplies the electrical department with its alternating current. The initial charge is obtained from a small Voss machine which is inclosed in the case of the Holtz. The photograph was obtained in the following manner. The machine was started and the brass knobs of the conductors adjusted to give a spark of about seven inches in length. The knobs were now tested in the usual way (by presenting a metallic point to the conductors) with reference to the sign of their charge. A gelatine dry plate was then taken, inclosed firstly in an orange and then in a black envelope. The plate was placed between the knobs of the conductors in a line parallel with them and the sparks allowed to play over the envelope for a period of one second of time. The plate was then taken to the dark-room, developed and fixed in the ordinary way. The accompanying illustration shows the curious results obtained. A distinct break can be seen in the continuity of the sparks between the positive and negative poles. Round the positive pole the sparks are rushing off in a dense mass with a direction from the negative pole of the machine. At the line of separation of this dense mass of sparks is seen a depression as if the mass had been eroded by the negative charge, reminding one very forcibly of what happens to the positive carbon of the arc light. At the negative pole the sparks are much less dense and more fan-shaped, and radiate in the reverse direction to the positive sparks with the exception of a cone of sparks, which are much smaller, which approach the depression in the positive mass. This prolongation of small sparks towards the positive pole is seen in each of the photographs obtained. The results of the experiment are curious. I am unable to explain them, but think they are perhaps worthy of record.