The clamp-type A-C microammeter

FOR MANY YEARS, the range of current-indicating instruments at power frequencies has been extended by the use of current transformers. A particularly convenient method is that in which the transformer consists of a secondary winding on a split toroidal core that can be clamped around the conductor whose current is to be measured. This technique suggests a similar device for measuring small currents at audio and radio frequencies: an instrument consisting of a small clamp-type magnetic probe and suitable means for amplifying the output of the probe to operate an indicating meter or an oscilloscope. Such an instrument is complementary to the a-c voltmeter in the audio- and radio-frequency range and has obvious utility in experimental work; this paper will demonstrate its realization.