PHOREOGRAPHY 7 the perfusion chamber beneath the skin, the other one is dipped in a little cup attached to the skin and filled with perfusion medium to assure a close contact. The surface of the measuring electrode is then determined by the diameter of the cup which is 1 cm. Procedure The intensity of current is increased stepwise within the range of 0-10 pA. The pulses of 1 s each are interrupted by intervals of equal duration, the intensity is increased to its maximum level within 90 s. This means 45 impulses yielding a profile as shown in Fig. 3. Then the maximum intensity is applied for 10 s followed by decreasing pulses (10-0) for another 90 s. Stimulation is said to be cathodic when the exploring electrode is con- nected with the negative pole of the stimulator, and anodic when the situa- tion is reversed. 190 • I00 -• 9o Decreasing stimulation Constant stimulation 0.2 0.4 0.6 0.8 1.0 ddp(v) Figure 3. Voltage profile out of the potentiometer (linear response of a 200K • resistor). In this paper we shall only consider the cathodic response, the general pattern of which is shown in Fig. 4. The critical point marking the change of the slope is called the threshold. The liminal intensity is defined as' 10xn I,= 45

8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Time (I cm/20 s) I I 6 5 4 3 2 I 0 0.5 I 1.5 /(/• A) v Figure 4. Skin response to a cathodic stimulation. where n is the number of pulses below the threshold. It is often beneficial to determine not only the liminal intensity but also the relative value of the decrease in resistance. Rx = below threshold skin resistance, R2 = skin resistance at a supra-liminal reference intensity IR, the ratio can be used as a term of comparison between measurement data from different subjects as it expresses the amplitude of the response to a stimula- tion of a given intensity. It has to be kept in mind that the threshold is a measure of the current intensity which is able to elicit the skin response. Its value reflects the sensitivity of the skin to the current which passes through it, and its value in gA cm-" is therefore a characteristic of the tissue. Skin resistance variation (AR) is an essentially relative value as it depends on the arbitrarily chosen reference intensity.