July 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Authors Cohen and Hessinger
warn about the need to consider the capacitive loading effects of shielded and closely-space test leads when measuring
other than direct current or very low audio or line frequencies. Lead capacitance is especially likely to affect
measured values when the frequency is high and/or the source and load impedances are high. As was common in the day,
capacitance units of μμfd (micro-micro farads = 10-6 x 10-6 = 10-12 F) are cited, which is
equivalent to units of pF (10-12 F).
One Problem in Choosing Test Leads
By George S. Cohen,* W8HTI, and Richard W. Hessinger*
Every audio enthusiast, amateur radio operator, and electronic
technician finds it necessary at one time or another to make some sort of electrical measurement. Many of these measurements
involve d.c. Bully for the man who has only this type of measurement
to make. But there are those of us who are not quite so fortunate,
for we must make many measurements on a.c. equipment. (This "a.c.
equipment" means anything other than d.c., and so involves a.f.
and r.f. as well as commercial power frequencies.)
Fig. 1 - The electrical circuit of any generator with an internal
impedance Zi working into a load ZL. A typical example is a vacuum
tube, where Zi becomes the plate resistance and ZL the load impedance.
Fig. 2 - Any practical voltmeter can be represented by the
meter M and Zi the impedance of the meter and its leads. Unless
considerably higher than ZL, the indicated voltage will be lower
Fig. 3 - When Zi and ZL have the values indicated, V = 0.375
Fig. 4 - If ZS is equal to ZL, the indicated value of
be 0.23 EG.
All of these measurements require some type of connecting leads
from the energy source to the measuring instrument. In the course of making many measurements
it was found that a source of difficulty, in one instance, was in
finding satisfactory leads for the particular job at hand. Although
the measurements of current and voltage present similar, if not
identical problems, we will consider voltage only.
Let us consider the generator of Fig. 1 with an internal impedance
Zi. This source is feeding a load with an impedance equal to ZL.
This circuit is very general and, as an aid in visualizing the problem, the
generator may be thought of as a voltage-amplifier stage in an audio
amplifier where Zi is the plate resistance of the tube and
the impedance the tube is working into.
If we assume that the generator is a pentode voltage amplifier
and that Zi is, therefore, of the order of 0.5 megohm and that ZL is in the vicinity of 0.3 megohm,
we will have a specific value of voltage, V, across the load.
If Zs is all capacitive reactance, as in this case, it will combine
with a resistive ZL of 0.3 megohm to give a resultant 0.21 megohm
and not the 0.15 megohm shown in Fig. 4.
Now, as is often required, let us measure the voltage across
the load impedance ZL. This circuit is-shown in Fig. 2. M is the
meter or measuring instrument and ZS is the combined impedance of
the instrument and its attached leads.
The voltage V that is measured in Fig. 2 will be approximately
equal to V of Fig. 1 only if the impedance ZS is ten or more times
greater than ZL. This happens because the shunting effect of the
impedance, ZS, combines in parallel with ZL to give a lower resultant
As an example, let us suppose that ZS is equal to ZL - 0.3 megohm
in this case. The parallel combination of ZS and ZL would then be
equal to 0.15 megohm. Fig. 3 shows that under the original conditions
V would be 0.375 times the generated voltage, EG. With the effect
of ZS the voltage across the combination would drop to 0.23 times EG, as
in Fig. 4. The difference in voltage is made up by additional drop
across the plate resistance of the tube.
Further, if this amplifier were operating at 2 kc. (a reasonable
frequency within the range of most audio amplifiers) then the shunt
capacity necessary to give ZS a value of 0.3 megohm would be 265
A value of 265 μμfd. may seem very large and unlikely to be
found in an ordinary pair of test leads. Recently, in testing a
circuit, we found that our calculations and actual results didn't
agree very closely and that started the usual search.
The culprit was soon found. A shielded multi-conductor cable
exhibited a capacity of 225 μμfd. from leads to shield and 128
μμfd. between leads. After this discovery, many leads in the
laboratory were measured. One of these was a 4 1/2-foot shielded
test lead supplied with an oscilloscope made by a well-known manufacturer. The lead measured 220 μμfd.
between shield and the center conductor. A compilation of the capacities
measured between other types of leads is recorded in Table I. An
examination of Table I shows that the experimenter must use good judgment in choosing an instrument
lead for voltage measurement.
There is a second consideration in choosing a lead and that is
noise pick-up of the cable. But we will not enlarge on this subject
There are, obviously, many more types of leads than are listed
in Table 1. The manufacturers of these usually list the capacity
per foot of each of the different types and other characteristics
that should be considered in using these cables.
We can do no more than point out one of the pitfalls and hope
that it has, or will at some later date, save you from excessive
blood pressure and loss of a few precious hairs.
* Commonwealth Engineering Co.
of Ohio, 1771 Springfield St., Dayton, Ohio.
Posted May 9, 2016