The Model 456A is an instrument for measuring ac current. It consists of a Probe and Amplifier and is used together with an associated voltmeter or oscil- loscope. Current is measured by merely clamping the Probe of the Model 456A around the current carrying conductor. The Probe operates with an accompanying small ampli- fier to convert the ac current being measured to a pro- portional voltage. The current-to-voltage conversion factor is 1 millivolt output for 1 milliampere flowing through the Probe Current readings can thus be taken directly from the voltage calibrations on the voltmeter or Oscilloscope