The Vreeland was a smooth oscillator, but it also had a digital rheostat or current control on the left side. This was not active during therapy; it was set once and left alone.
Here’s a clearer picture of the same digital rheostat in use with a forge:
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Jerky current control was just as bad as jerky oscillation, when the current controller had to be moved during therapy.
Water rheostats became popular in the 1800s for two important reasons. First, water can’t be burned. It can boil off, but steam tech had mastered the materials and techniques of keeping water refilled. Second, water is perfectly smooth and analog. Most heavy current controllers, then AND now, are digital, with a switch selecting big resistors of different sizes, or switching in more windings on a motor.
Water rheostats were often used in situations where smooth control of HEAVY current was needed. Here Happystar is regulating the current to a Cottrell dust collector as Polistra watches for just the right level of performance.
Pure water has a high resistance, and changing the amount of water between the two conductors changes the total resistance smoothly.
Water resistors were popular in electrotherapy devices. This is the Lockey coil, just a buzzer and induction coil to provide a high-voltage charge, plus a manually variable water rheostat to control the output.
The Lockey was then extended to the ultimate use of a water rheostat. The Enallax-Ohm was a programmable and smooth variable current source. The water rheostat was driven gradually up and down by a clockwork mechanism that could be set for various speeds and deltas.
Here Polistra is getting some slow and smooth muscle exercise, to strengthen a muscle after long disuse.
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Another source of soothing and smoothing is warmth. I’m highly aware of this modality after shoveling and deicing the sidewalk. The sun finally popped out of the clouds just now, and I let it bathe and saturate my tense neck.
The Oudin was an induction machine designed to generate higher frequencies. Those higher frequencies turned out to be the most useful and universal form of electrotherapy.
The Oudin’s high frequency output led to a discovery. Higher freqs can penetrate tissue, making it possible to heat up a tense muscle, or create a localized internal fever to burn up microbes.
The simplest way to get controlled heat is with steam radiators or simple resistance heaters inside a box. This device is familiar from old cartoons of weight-reducing salons. (Was it really used that way? I doubt it.)
High frequency sources based on the Oudin self-induction coil became hugely popular for many decades. This diathermy machine is working on a stiff knee joint:
In the 1920s diathermy machines, for medical and industrial heating, outnumbered radio stations. A history of the first station in Oregon tells us: Diathermy machines were another constant irritation, and owners of these were ever in conflict with radio listeners.
Better tuning and frequency allocation solved the interference problem. Later diathermy moved up into the microwave range, and now nearly every house has a diathermy machine, used primarily to heat food.