Peak Voltage Measurement in Diathermy

Abstract

IN the heating of tissues by high frequency diathermy currents, a milliammeter in series with the subject receiving the treatment records the current passing between the electrodes. The current is adjusted by the spark gap or by the impedance in the primary circuit of the 60-cycle transformer to a value sufficient for the dosage. In practice, it is assumed that the current as indicated by the milliammeter per unit area of electrode will give an indication of the temperature increase of the tissues below the electrode. Recently Kelley (1) has asserted that it is voltage rather than current which determines the heat production below the diathermy electrode. Sampson (2) states that “the high voltage current has a much greater heating value, milliampere for milliampere, than the low one.” It is with the intention of investigating quantitatively the effects of voltage variation on heat production in diathermy that the following experiments were undertaken. In diathermy it is necessary to distinguish between two types of voltage, namely, “peak voltage” and “root mean square voltage.” The following diagram (Fig. 1), which is purely schematic and not drawn from an experimental record, will illustrate peak voltage. The low frequency (60 cycles per second) voltage is “stepped up” by the low frequency transformer to a high voltage of low frequency which is impressed on the condensers. This is represented in Figure 1 by the Curve A. When the voltage on the condensers reaches a value sufficient to produce sparking at the spark gaps, a spark discharge at the gaps occurs and the condenser low frequency voltage drops (see B in Fig. 1). At the same time an oscillating current of high frequency similar to C is produced in the oscillating circuit containing the air core autotransformer, high voltage condensers, and spark gaps. This high frequency current, whose frequency is determined by the capacity and inductance of the oscillating circuit, passes through the patient. The resultant voltage on the condenser at any instant then will be given as the sum of voltages similar to B and C. The peak value corresponding to D of the oscillating voltage applied to the patient will be referred to as the “peak voltage.” The current passing through the patient also has a wave form similar to the voltage of Figure 1. The current registered by the meter, however., records the “root mean square” current, being the square root of the average of the squared values of the instantaneous current. Corresponding to the “root mean square” current there is a “root mean square” voltage, which also gives an average effect of both positive and negative halves of the voltage cycle. In Figure 2 is given a circuit to measure this peak voltage. The circuit to the left of the calorimeter represents the usual diathermy circuit with low frequency transformer, high voltage condensers, spark gaps, and air core autotransformer.