E.3.8 Hysteresis Curve

The circuit below, from Physics Demonstration Experiments Volume 2 by Harry Meiners, page 972, will draw a hysteresis curve on the oscilloscope. The twenty ohm resistor serves to measure the current to the transformer primary producing a horizontal signal proportional to H. The output of the secondary of the transformer is proportional to dB/dt. The final RC circuit integrates this (see E.7.5) to produce a vertical signal proportional to B. As you adjust the variac to control the current to the primary, the curve on the oscilloscope stretches out to saturation.