The Sechometer

Polistra is cranking a strange little machine, and the galvanometer needle is wiggling in response.

The mysterious little gadget is a sechometer, with an equally mysterious name. It was made by Leeds and Northrup, one of the two big lab equipment companies. I wrote a series of features on L&N in 2021. Allegedly the name was misspelled from secohm, the older name for the unit of inductance now called the Henry. It was fairly common in industrial and academic labs around 1920.

The sechometer made by L&N looked sort of amateurish.

It was just a little wooden box with a crank on one side and a shaft with pulley and flywheel on the other. Inside, the shaft drove a pair of commutators, similar to the commutator in a motor or generator, but without any magnetic parts. The commutators connected up two terminals either forward or backwards. The crank turned the commutator through a speedup gear, and the pulley allowed a motor to drive it more accurately.

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Why did labs want a sechometer? One way to check the performance of a resonant circuit is to give it a quick pulse or transient and see how it responds. A high-pass filter will emphasize the transient, and a low-pass will damp it. A bridge arrangement was commonly used for such tests, providing a strong indication of small differences from a standard component, important in commercial situations. Industrial labs wanted to automate this process, repeating the pulse over and over at controlled frequencies to test the response of one component.

The sechometer provided a steady set of pulses in both directions by simply switching the polarity of the battery back and forth repeatedly. At the same time it switched the input to the meter back and forth, so the meter would tend to average the response instead of canceling out the opposite directions.

Later developments made this trick unnecessary. Observing the response on an oscilloscope, or the digital version of a scope, gives the full picture of back-and-forth transient response.

We need to see the sechometer in schematic form to understand what it was doing. Here’s a typical bridge arrangement for testing one capacitor against a standard capacitor.

Typically, sliding the potentiometer back and forth would find the point of balance between the two sides, indicated by zero on the meter. In this setup we’re not really looking for a null point; we’re trying to stir up the difference between the sides by applying a pulse.

Each commutator is a DPDT reversing switch. They always flip together, so that applying the battery forward sets the meter to read forward, and applying the battery backward sets the meter to read backward.

In more modern terms, the sechometer was a square-wave oscillator and a rectifier working in tandem. It would be replaced soon by vacuum tube circuits, and then by the oscilloscope.