Radio World Ad for Fil-Ko-Stat, 1929

SUPREME because it was designed to be supreme! shouts the ad copy in this back-cover ad from Radio World magazine. Flying majestically above the setting (or is it rising?) sun, we see a genuine Fil-Ko-Stat.

What is a Fil-Ko-Stat, you say? Judging from the text, it is a 30-ohm rheostat (variable resistor) designed to regulate the filament voltage of a radio tube. The text encourages you to hook up one of these to every tube in your radio, and the illustration shows a Fahnestock clip on each end of the device.

I'm not sure why one would want to individually adjust the filament voltage for each tube. Increasing the filament voltage generally increases a tube's emissions, but most radios (more modern ones, anyway) operate at a fixed filament voltage and vary the tube's output in more sophisticated ways.