Violet Ray / Violet wall device (No. 4) Dynamotherm. Early 20th century. With 10 electrodes. These have been tested and seem to work (give light). One electrode doesn't seem to work, it has a spiral inside, see photo 4. This set has a lot of special electrodes. Please see the photos. The rubber tubes of some electrodes have decayed. The device has been tested and works. See the photos, they give a good impression of the condition of this device. They are an integral part of the description thereof. A violet ray is an antique medical appliance used during the early 20th century to discharge in electrotherapy. Their construction usually featured a disruptive discharge coil with an interrupter to apply a high voltage, high frequency, low current to the human body for therapeutic purposes. Their basic construction was invented before 1900 by Nikola Tesla, who introduced his first prototypes at the World's Columbian Exposition in 1893.