Measurements on impulse voltages with a ballistic galvanometer

Simple Circuits are described whereby measurements of the crest value and time of duration of the transient voltages used in impulse testing may be made with a ballistic galvanometer. Both methods of measurement have been compared with the results obtained simultaneously by a high-speed cathode-ray oscillograph. The agreement in crest voltage was within 2%, even with voltages having a duration of the order of 10¿7 sec., and in time-lag was within 0.2 micro-sec. when the grid bias of the valve was suitably adjusted. The measurements are not critically dependent on the characteristics of the thermionic valves used in the circuit, and the range of voltages which may be measured can easily be extended by the use of suitable high-voltage resistors. The two circuits have been assembled conveniently into a general impulse-measuring equipment with mains-operated supplies for the valves. As an example of its use the time-lag curve of a 10-in, rod gap, obtained in a small fraction of the time necessary to do so by oscillographic methods, is reproduced. The paper concludes with a few measurements of the length of the front and tail of an impulse wave which have been made with the equipment.