Observations placeholder

THE METAL-BENDERS” by JOHN B. HASTED

Temperature is a measure of the mean of the random speeds of motion in solids, liquids and gases, so that the elevation of temperature may not itself represent a primary physical phenomenon. For example, a shock wave produced by impact dissipates its energy in the form of a rise in temperature, and the action of a compression piston on a gas has a similar effect.

The normal bending of a metal is accompanied by a local change in its temperature, and the stretching (back) of such materials as rubber is accompanied by a fall in temperature.

Resistive strain gauges are temperature-compensated; that is, they are designed so that the resistance is invariant to small changes of temperature. Therefore our dynamic strain signals are not due simply to small changes of temperature. However, the dynamic strain pulses are probably accompanied by minute local temperature changes, to which the gauge would not respond.

There are well-known devices, including thermistors, whose electrical resistance is very sensitive to temperature, but very much less sensitive to strain. If we wish to investigate the possible existence of temperature changes during metal-bending, such devices can be attached to metal specimens and chart-records made of the time variation of their resistance.

I carried out such experiments with Stephen North, using the temperature gauge type STG 50D made by Micro-Measurements Inc. The sensor, which is formed of a nickel filament enclosed in epoxy-resin and glass fibre, has a nominal resistance of 50 Ohm and was used with resistive compensation in our standard 120 Ohm arm bridge. Resistive strain gauges were also mounted on the metal specimen, and strain pulses were recorded during the sessions, including a permanent bend.

I have at no time in these sessions observed any sudden temperature change at a thermal sensor. The high thermal conductivity of the metal specimen makes it possible to detect temperature changes over an area rather wider than that of the thermal sensor, whose dimensions are 12 X 9 mm. The sensitivity of the equipment is such that changes of less than one-fiftieth of a degree could be detected. Thus the proximity of the subject’s hands, and local air currents, were both detectable as relatively gradual temperature changes.

Stephen North soon understood the magnitude of these effects and succeeded in keeping sufficiently far from the sensor to avoid serious interference. No dynamic signals of any significance were recorded on the temperature charts during these sessions, although strain pulses were recorded. I have not pursued thermal sensor studies in extenso because of the initial absence of signals; but it seems unlikely that anything more than highly localised temperature effects are produced by metal-benders.
However, this is not evidence that there are not minute local secondary temperature changes occurring when the resistive strain gauges receive dynamic strain pulses. Italian physicists have reported macroscopic temperature rises in specimens stroked manually by metal-bender Orlando Bragante.