For a long time the design engineer in the laboratory or the technician in the iield has been in need of an instrument that would measure dc currents without breaking the leads. Since the introduction of the transistor, which is a current device, the need for a fast, convenient method for measuring dc currents became even more apparent. The Model 428A Clip-On Milliammeter realizes the above stated objectives in a unique way. The instrument measures the magnetic field, which exists around the wire carrying dc current. Operating the instrument is simple. After zero setting, the two jaws of the probe are clamped around wire (arrow on probe head indicates direction of conventional current flow) and the meter will indicate the current. There are six current ranges starting from 3 ma to 1000 ma full scale deflection. The sensitivity can be increased even further by looping the wire several times through the opening in the probe. For example, three turns increase the maximum sensitivity to 1 ma full scale. The current indication is virtually insensitive to superimposed ac signals and the series loading of the circuit is less than 0.5 µh. A large amount of feedback provides great stability. With the 428A, currents can be measured as easily as measuring voltages with a voltmeter.