High impedance electrical testing instrument for AC and DC voltage detection and continuity testing

Claim 1

An electric testing instrument comprising: a housing having a wall of electrically conductive material adapted for manual engagement by the user and forming an input terminal for completion of an input circuit through the body of the user; an amplifier mounted in said housing and having its input connected to an input circuit comprising a probe providing a second input terminal adapted for engagement with an electric signal, said amplifier having an output circuit comprising in series a collector, an emitter, a battery and a current responsive indicator; said housing wall being connected to the side of said battery connected to said collector and having polarity opposite said emitter; said indicator providing a manually discernible signal at and above an operating current flow in said output circuit, said amplifier having a gain sufficient to provide said operating current flow in said output circuit when said input circuit is biased with an input current; said input circuit providing said input current derives solely from said battery when said input terminals are connected across a resistance from zero ohms to as high as at least one megohm; an electrically nonconductive material covering a portion of said housing wall; said amplifier having a sufficient gain to provide said operating output current flow when said probe is connected to a conventional live power line providing approximately 110 volts a.c. and said nonconductive material is manually engaged by the user; and said material covering a part only of said wall leaving an uncovered portion for direct manual engagement by the user for through the body continuity testing and for detecting the presence of d.c. voltage, thereby enabling the user to distinguish between a.c. and d.c. signals.