GB611710A - Improvements in radio frequency heating, particularly for the permanent waving of hair - Google Patents

Abstract

611,710. Heating by electromagnetic induction. WARREN, S. F. Oct. 10, 1944. No. 19444. [Class 39 (iii)] Also in Group XXXV] In radio frequency heating apparatus, particularly for the permanent waving of hair, heat is generated by electro-magnetic radiation producing a curculating current in a tuned oscillatory circuit comprising an element the electrical characteristic of which varies with temperature, whereby the tuning of the circuit and consequently the heat generated varies with variations in the temperature. A substantially constant predetermined temperature may be obtained by employing in the oscillatory circuit a condenser that employs as dielectric a material having a dielectric constant which varies with temperature, e.g. the ceramic material known under the Registered Trade Mark "Faradex," which detunes the circuit automatically when the predetermined temperature is exceeded, and restores tuning after a requisite amount of cooling. The condenser is located in close proximity to a resistance heating coil connected to it in series with a low impedance pickup coil in the oscillatory circuit. Since the heating device heats up slowly when the condenser is off tune, means may be provided for adjusting manually the tuning of the oscillatory circuit, preferably to two or more pre-set conditions, or the heating devices may be pre-heated by heating separately in a warming box or chamber before the pick-up is submitted to R.F. radiation. Preferably, however, the transmitter is adapted to have the frequency varied, either continuously, or in stages to two or three pre-set values, until the final frequency is attained at which the desired temperature is reached. These frequency changes may be effected automatically by an electric motor. The circuit comprising the low and high impedance coils may be embodied in a system for the permanent waving of hair, by embedding the heater coil in a ceramic material formed with a vitreous surface to facilitate cleaning. The ceramic may be of tubular form with the heater coil embedded in it, and arranged to be slid over a wound tress of hair on a curler, or it may be constructed in the form of a clamp. The pick-up coil may be carried by the ceramic, 'preferably in a mechanically adjustable manner so that it can be disposed in the position to obtain maximum pick-up from the radiating loop. Alternatively the pick-up coil may comprise two or more coils arranged at different angles. Instead of the position of the pick-up coils being adjustable the transmitter may comprise two or more loops at different angles. The transmitter may be provided with a switch whereby its output may be varied in steps, e.g. by changing the voltage applied to the anode of the output valve, and when a number of heating devices are employed, as in the permanent waving of hair, each heating device may be provided with an indicating member such as a lamp which operates when the heating device is energised. Specification 569,631 is referred to.