Diathermy machines, infra-red radiation, hot compresses, etc., have been used for many years for the production of heat in local parts of the body in the treatment of disease, particularly arthritis. In the case of heating lamps, the penetration of heat is so slight that the temperature of the deep parts is not appreciably raised. In cases in which heat is transmitted by conduction, as in hot compresses, the dissipation of heat by blood flow through the part treated makes it impossible to administer heat to the deep structures. With diathermy machines, in a case in which a high frequency current is passed through the part by means of electrodes directly applied to the skin, there is produced a through-and-through heating, and such machines have sufficient power to overcome the dissipation of heat by blood flow, etc., and to maintain the elevated temperature of the part as long as desired. However, it is difficult at times to attach electrodes to certain parts of the patient, such as the shoulders, and there are certain difficulties in securing good contact with the skin. Through recent advances in the radio field it has become possible to apply energy to a part of the body and cause a rise of temperature in the tissues without an actual application of electrodes; in other words, we now have an apparatus at our disposal which retains the advantages of a diathermy machine as far as deep heating is concerned, but with many of the disadvantages eliminated. It can be constructed in any well equipped laboratory and need not then cost more than an ordinary diathermy machine. Carpenter and Page (1) describe a radio apparatus of high power for the raising of the systemic temperature. Pätzold (2) describes a lower powered machine for the local application of this form of energy. However, no circuit and but few data are given. We propose to describe in detail an apparatus constructed in this laboratory and which has been applied clinically for some time with good results. If the essential circuit requirements are complied with, there is considerable latitude in the arrangement and design of the apparatus, as well as the manner of application of the energy to the patient. The machine consists essentially of a high frequency vacuum tube oscillator, operating at a frequency of about 10,000 kilocycles. The output of the machine is led to two insulated plates between which the part of the body to be treated is placed. An intense, rapidly fluctuating, electrostatic field exists between these plates, and the heating effects are brought about by electrical losses, such as dielectric hysteresis and eddy current losses, occurring in the part of the body under treatment. The principal circuit (Fig. 1) is essentially that of a short wave radio transmitter. Two UX 852 radiotrons, rated at 75 watts each, are connected in a push-pull Hartley circuit. The tubes are energized by a 2,000-volt, full wave rectified but unfiltered, power pack.