Untitled Document

08 August 2003

The Conductivity of Insulators

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published October, 1969.

Occasionally an unexpected event such as the flick of a pointer across a dial, a puff of smoke or a flash of light alerts the careful experimenter to a fruitful opportunity. A case in point is reported by W. W. Withrow of Teague, Tex., who recently hit on an inexpensive method of measuring the electrical conductivity of materials ordinarily classed as insulators. He writes as follows:

"Some months ago I received a number of transistors from a cousin who works for a computer manufacturer. The devices had been rejected during tests at the plant. They were designed to conduct current from the emitter to the collector terminals but not in the reverse direction. I decided to measure their resistance in the reverse direction, on the assumption that those of high resistance might be usable. The measurements were made with an inexpensive ohmmeter that has a midrange scale of 150,000 ohms.

"During all the tests except one the pointer of the meter swung to the top of the scale and stayed there, indicating a usable transistor. In this one case, however, the pointer first swung to the top of the scale, flickered a few times and finally dropped close to zero. Initially I thought the device was defective. Then I became aware that the tip of one of my fingers was touching both the collector and the base terminals of the transistor. When I removed the finger, the pointer swung to the top of the scale, as in the case of units previously tested. Evidently current through my finger was triggering the transistor into its amplifying state. The relatively small current in the base-collector circuit caused a large current in the emitter-collector circuit. It occurred to me that the transistor might be used to magnify the scale of my ohmmeter, perhaps from its normal midrange of 150,000 ohms to several million ohms.

"To test this notion I clipped the positive terminal of the meter to the emitter lead of the transistor and the negative lead of the meter to the collector. (The silicon transistor was of the p-n-p, or positive-negative-positive, type.) I then connected a one-megohm resistor across the collector and base terminals of the transistor. The pointer of the meter promptly swung to 10,000 ohms, which suggested that the transistor had magnified the scale of the instrument 100 times, equivalent to a midrange value of roughly 15 megohms. The thing had workedl

"Why not place another transistor ahead of the first one and get still more magnification? This was done by connecting the base of the first transistor to the emitter of the second one and interconnecting the collector leads. In this circuit the transistors function as a two-stage current amplifier [see illustration below]. The circuit again worked, amplifying 10,000 times to yield a midrange scale of 1,500 megohms.

Circuitry of W. W. Winthrow's amplifier. Click image to enlarge.

"Rough measurements are of little use. My next step was to calibrate the magnified scale, which can be done with two resistors, one having a known, fixed value and the other being variable from zero to at least 1,500 megohms. The resistor of known value is connected in series with the variable resistor. The variable resistor is set to zero and the fixed resistor alone is measured. The position to which the pointer swings is noted. The fixed resistor is short-circuited by connecting a copper wire across its terminals. The variable resistor is adjusted so that the pointer swings to the previously noted position. With the pointer at this position the value of the variable resistor matches that of the fixed resistor.

"The value of the fixed resistor can now be added to the circuit by removing the copper wire. The pointer then swings to a new position that is equal to twice the value of the fixed resistor. With the fixed resistor again short-circuited the variable resistor is adjusted so that the pointer swings to the new position on the scale. The cycle of operations is repeated until the expanded scale is fully calibrated.

"Fixed resistors calibrated to within 1 percent of their rated value are available from suppliers in sizes up to several megohms. Obtaining a variable resistor of the size needed for this job is another matter. Variable resistors of zero to 3,000 megahms are not available commercially. I tried a number of ways to improvise one with several materials in my shop.

Details of the variable resistor. Click image to enlarge.

"One of my devices seemed promising. It was a wood scrap two inches wide and two feet long with a series of steel nails driven in a row down the middle to serve as terminals. The strip had about the right order of conductance, but the conductivity changed faster than I could clip instrument leads to the nails. Evidently current in the wood was carried by absorbed moisture.

"After trying a number of other materials without success I finally hit on the idea of using a cadmium sulfide photocell as a light-dependent resistor. I sealed the photocell in a small, lightproof box along with a miniature lamp bulb, a flashlight battery and a rheostat for adjusting the brightness of the lamp with a knob outside the box [see illustration right]. The maximum resistance of the cell in darkness is about 3,000 megohms. All photocells of this type are sluggish: their resistance continues to change for several seconds after each adjustment of the light. Their resistance also changes with temperature and other variables. Even so, if one is patient, the device can be used to calibrate the ohmmeter. Any type of silicon transistor with a current gain (beta) of 100 or more will work in the ohmmeter circuit.

"Resistances up to a billion ohms are normally measured with a 'megger,' essentially a hand-cranked generator, that develops several hundred volts. It is connected to a meter that responds to a few millionths of an ampere. Meggers are costly and inconvenient to use. The same measurements can be made by fitting a $20 ohmmeter with a pair of silicon transistors. I use mine for checking capacitors, insulators and similar electronic components."