Oscillographic study of the current and voltage in a permeameter circuit

The purpose of the investigation was to study the form of the voltage-time and current-time curves, existing in a permeameter circuit, and to reduce the time required for the reversal of the magnetizing current. The permeameters used in the investigation were of the U-shaped yoke type. Oscillograms were taken of the current and of the induced voltage during the opening and also during the reversal of the magnetizing current. The permeameter, with which the investigation was started, was fitted with brass end pieces to support the magnetizing coil. The oscillograms showed that the flux change lagged behind the magnetizing current. In fact, the secondary e. m. f. continued for about one second after the current change was completed. The cause of this lag was found to be due to eddy currents set up in the short-circuited paths provided by the brass end pieces. After these were removed oscillograms showed that the lag in flux behind the magnetizing current was negligible. This brought out clearly the fact that short-circuited paths in which eddy currents may be induced should be avoided in permeameter construction. Two new permeameters were then constructed of the same type; one of these was made with a solid core of silicon steel and the other with a laminated core of the same material. Tests of these showed that use of the laminated core materially reduced the time required for the reversal of the current. Several different types of switches were used for opening and reversing the magnetizing current. The oscillograms showed clearly that a quick-break snap switch operating under oil is superior to other types of switches.