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HEWLETT-PACKARD
JOURNAL
T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p - L A B O R A T O R I E S
Vol.9 No. 10-1
PUBLISHED BY THE HEWLETT-PACKARD COMPANY, 275 PAGE MILL ROAD, PALO ALTO, CALIFORNIA JUNE-JULY, 1958
A Clip-On DC Milliammeter For Measuring
Tube And Transistor Circuit Currents
IN typical electronics work direct currents in practice often results in omission of the
are ordinarily measured either by measur measurement with subsequent erroneous de
ing the current directly with a moving-coil duction and wasted effort. Additionally, in
type current meter or by measuring the volt the continually-growing field of low- imped
age drop across a known resistance in the ance circuits, where the current parameter
circuit. Although each of these methods has assumes greater importance as in transistor
a long tradition, each also has substantial work, the resistance of the moving-coil in
disadvantages. In the case of the indirect strument is sometimes intolerable.
method with a voltmeter, the A new and much-easier-to-use current-
measurements are most often measuring instrument that overcomes the
made across resistances hav foregoing objections has been developed in
ing tolerances of 10% to 20% so that a mini the form of a clip-on dc milliammeter which,
mum ambiguity of this same amount also as far as is known, is the first instrument of
exists in the current value obtained. Fre its type to be brought to production status.
quently, this ambiguity leads to selection of It makes the measurement of direct currents
non-optimum component values with later in low- as well as high-impedance circuits as
difficulties. In the case of the direct method easy as a measurement of voltage and at the
with a moving-coil instrument, the circuit same time covers the whole range of currents
must be opened and the current meter con encountered in such work