I have not yet been able to date this unit, at an estimation it would have been made around 1907 and uses a 1.5V #10 battery often used for clocks. These are also called a type TA battery.
They were a square battery that had two screw terminals on the top, the center was positive and the one on the edge was negative.
When the unit came to me it was not functioning, the interrupter was bent and there was a connection issue with one of the terminals. I have managed to repair these and the unit works a treat.
The next phase is to rebuild the probes and cables and make a battery for the unit.
The probes used for most of these are a brass cylinder with a wooden handle, the cable is usually cotton coated and often resembles a thick litz type wire with really thin strands.
When the unit is running the interrupter vibrates sue to the electromagnet that can be seen on the top as the green winding, Power flows in to the electro magnet and breaks the contact causing what seems to be back EMF through the main larger coil inside, this spike creates an inductive kick that stimulates the muscle that the probes are connected to.
One interesting thing about the vintage units is that they mostly rely on an inductive kick rather than voltage and stimulate the muscle directly whereas more modern TENS units seem to rely on a higher voltage to stimulate the nerve in to triggering the muscle.
Over the years of collecting these units and dissecting how they work I have found that they are a lot less painful to use than the modern counterpart and really do promote blood flow in the area being treated.
The current output of the spike seems to be around the 30 ma mark with a voltage that is usually between 2 to 4 volts.
At present I do not have access to a scope to this has been tested using a multimeter so it is possible that the readings are not overly accurate and I am missing some other spikes being produced.