Seventy-five years ago this month, the March 1944 issue of Radio News reminded readers that they could use a broadcast radio to make sure their 100 kHz crystal calibrator was in tune.

Before the days of digital readouts, you needed to know what frequency you were tuned to. The easiest way to do this was with a crystal calibrator, an oscillator putting out a signal on (usually) 100 kHz. To calibrate your receiver, you would switch on the crystal calibrator, and you would hear the harmonic every 100 kHz up the dial.

To make sure the calibrator was in tune, you could use the method described in this article. You would hook the calibrator up to your broadcast receiver, and then zero-beat it with a broadcast station on a frequency divisible by 100. The article included a list of the 108 stations in the U.S. that met that description. (The article noted that 83 of those stations were on 1400 kHz.

The article noted that FCC rules required broadcast stations to be within 20 cycles of their assigned frequency, although most were closer. It also suggested that a phone call to the station might get a more exact answer as to how close they were at that moment.