Monday, January 6, 2014

Heathkit and Old Electronic Measurement Units

A note about measurement units, as this can be a possible area of confusion with older electronic equipment. As an American company, Heathkit catalogs and manuals generally gave dimensions, weights and other measurements in what are now known as United States customary units (i.e. inches, pounds, degrees Fahrenheit). Later catalogs, particularly the Canadian versions, would often also list metric units (Canada officially adopted the metric system in the 1970s).

Over the years, the units used for various electrical properties have changed somewhat. The Hertz was officially adopted as the unit of frequency in 1960 but was not consistently used until the 1970s. Newer Heathkit manuals and catalogs would therefore use hertz (Hz), kilohertz (kHz), megahertz (MHz), and gigahertz (GHz). Prior to that, frequency was usually measured in the equivalent unit of cycles per second (cps), and sometimes just "cycles". Common units were kilocycles (KC) and megacycles (MC). The front panels of Heathkit instruments reflect the units that were common at the time. Very old radios sometimes display wavelength in meters rather than frequency, on their dial scales.

The units of resistance, capacitance, and inductance have been standardized as metric units from the beginning. However, for capacitance, the unit nanofarad (nF) for 109 Farad was not commonly used in North America until recently, and the picofarad (pF) for 1012 Farad, was commonly referred to as the micromicrofarad (µµf) in some older manuals and catalogs. You may also see references to the word condenser. This is simply an older name for capacitor.

In my book, Classic Heathkit Electronic Test equipment, I generally use the modern units, but when listing product specifications, they are reproduced exactly as in the original Heathkit manual or catalog entry, in some cases using the now obsolete measurement units.

Incidentally, some old schematics sometimes used M as an abbreviation for thousands rather than the more common modern usage meaning millions. For example, a resistor labelled as "10M" may in fact mean 10,000 ohms rather than 10 million ohms. This usage seemed to occur in the 1930s and early 1940s and I have not seen it in any Heathkit-written documents or manuals.

The above is based on material from my book Classic Heathkit Electronic Test Equipment, available from lulu.com  and amazon.com.

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