In the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine.
Ultrasonic diathermy refers to heating of tissues by ultrasound for the purpose of therapeutic deep heating. No tissue is ordinarily damaged hence it is generally used in biomedical applications.
Electric diathermy uses high frequency alternating electric or magnetic fields, sometimes with no electrode or device contact to the skin, to induce gentle deep tissue heating by induction. Again, no tissue is ordinarily damaged.
Surgical diathermy is usually better known as "electrosurgery." (It is also referred to occasionally as "electrocautery", but see disambiguation below). Electrosurgery and surgical diathermy involve the use of high frequency A.C. electrical current in surgery as either a cutting modality, or else to cauterize small blood vessels to stop bleeding. This technique induces localized tissue burning and damage, the zone of which is controlled by the frequency and power of the device. Some sources insist that electrosurgery be applied to surgery accomplished by high frequency A.C. cutting, and that "electrocautery" be used only for the practice of cauterization with heated nichrome wires powered by D.C. current, as in the handheld battery-operated portable cautery tools.
Medical Diathermy devices were used to cause interference to German radio beams used for targeting night time bombing raids in WWII during the Battle of the Beams.
- Valleylab article on Principles of Electrosurgery/Electrocautery