Keith Wilson - Electrical engineer
When you next power up your trusty ohmmeter, it’s unlikely that your intention will be to measure the resistance of a lump of pork or bacon. Things were rather different back in 1935, however, when Evershed and Vignoles, one of the predecessor companies of Megger, sponsored the publication of a paper with the somewhat surprising title “The Electrical Resistance of Pork and Bacon”.
So what was it all about? A bored development engineer who just happened to plunge his test probes into his breakfast sandwich and then decided to share the results with the world? No! As you might expect, there was a very good reason for writing the paper and for carrying out the research that underpinned it.
In an effort to justify this not inconsiderable effort, the paper points out that meat had been preserved by curing for many years, but that – back in 1935 at least – there were few scientific controls over the curing process. In essence, the meat was pumped with a suitable pickling fluid, then placed on a dry-salt stack or placed in a pickling tank. The only guide to when the curing process was complete was experience or to cut off a small piece of meat and taste it, which gave a very rough guide to the salt content.
Neither of these options was particularly satisfactory, but it was important to ensure that the meat was properly cured otherwise it would spoil and quickly become unfit for consumption. Bear in mind that few homes at this time had refrigerators, so domestic purchasers at least were heavily dependent on the proper processing of meats to guard against the risk of food poisoning.
To address this issue, the author of the paper, F H Banfield, decided to investigate whether measuring the resistance of the meat would provide a reliable indication of salt penetration and hence the progress of the curing process. His first experiments were carried out with a supply derived from the AC mains and rectified with a thermionic valve. The current in the meat was measured with a milliammeter and the resistance calculated using ohm’s law.
The results were less than satisfactory, as electrolysis meant that chlorine was liberated at the test probes and this discoloured the meat. For subsequent experiments, a valve oscillator running at 1 kHz (1,000 c/s in those days!) was used, and the resistance measurements were carried out with a modified version of a Wheatstone bridge.
Now the results were encouraging, showing a definite and repeatable, albeit non-linear, relationship between the resistance of the meat and the salt content of the pork. So far, so good, but using a valve oscillator and a Wheatstone bridge with headphones as null detector was quite clearly not exactly an optimum solution in a busy meat processing plant.
Evershed and Vignoles therefore designed a much more practical method of measuring the resistance of the meat and, with startling originality dubbed this the Meg Salinity Tester. It probably won’t come as much of a surprise that this was based on the popular hand-cranked Megger insulation tester of the period.
To quote the official description, “The instrument consists of a hand generator and ohmmeter combined in a one case. It is used in conjunction with a special probe, the principle of operation being shown in the diagram. The deflection of the ohmmeter movement, which depends on the ratio of the currents in the current and potential coils, gives the resistance directly. The rotary converter and the rectifier enable alternating currents to be used in the testing circuit, thus eliminating electrolytic action at the electrodes.”
Unfortunately, the paper gives no indication of the commercial success or otherwise of the Meg Salinity tester so it’s impossible to say whether these instruments were produced in quantity and widely used, or whether they remained little more than a curiosity.
The whole concept of the tester does, however, conjure up a rather appealing visual image of an inspector at a pork processing plant wandering around and occasionally probing a piece of meat while cranking an instrument that must have looked for all the world like a Megger insulation tester!
(Grateful thanks to the Archive Department of the IET, which provided access to the paper on which this article is based.)