244 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS and 1'0 g of hair, in view of the fact that in this region the charge is approxi- mately independent of the weight of hair (Fig. 8). The general shape of the curve shows that the charging operation becomes more difficult as the charge on the hair or comb increases. These tend to become saturated with charge after a certain number of strokes. The variation of charge with relative humidity Using one gram locks of hair and the standard shampoo technique, the charge was determined on different samples of hair at different relative humidities. Over the middle range of humidities the charge was found to decrease linearly with increase in relative humidity. u,) 30 20 i I I0 20 ß ß 30 40 50 60 70 BO 90 O0 % R.H. Fig. 5. Variation in charge with Relative Humidity. 1 g samples of hair washed in a standard shampoo and tested at different humidities using ten hair strokes. It is interesting to note that the relative humidity at which there is virtually no charge, i.e., 85 per cent, corresponds roughly to the point on the adsorption isotherm of wool keratin at which multilayer formation begins to occur 6. At this point surface conductivity could be expected to be maximal and the static charge minimal. The effect of earthing the hair Four samples of hair were given the standard shampoo treatment with two different types of shampoo. Each sample was tested (a) earthed in the normal manner, and (b) with the earthing wire removed. The results are shown in Table 8.

STUDX2'ING STATIC ELECTRICITY PRODUCED ON HAIR BY COMBING 245 Table 2 The mean charge and standard deviation on four 1 g samples of hair, with and without an earthing wire attached to the hair I Not Earthed S.D. Earthed S.D. Shampoo A -- 1 8.5 0.6 10.6 0.9 2 13.7 1.1 12.1 1.2 -- Shampoo 13 -- 1 11.8 1.3 10.8 1.4 2 11.0 1-4 12.0 2.5 There is no significant difference between the results obtained, with and without the hair being earthed. A comparison of the charge produced after shampooing with a number of types of shampoo Samples of hair were washed with five shampoos, each being based on a different type of commercially available alkyl sulphate. The charges on three of them were determined at three different relative humidities. The shampoos were compared on an equivalent basis of the active deter- gent ingredient. The results are shown in Table 8. Table $ Comparison of the magnitude of the charges produced after shampooing with five different types of alkyl sulphate shampoo R.H. 40% 60% 70% Mean S.D. Mean Mean Shampoo Charge Charge S.D. Charge S.D. __-- -- A 18.5 2.17 16.0 1.64 10.2 1.31 B 20.3 1-50 15-2 1.99 9.6 1-14 C 19.4 1.78 13.0 1.75 9.6 1.23 D 18.5 ..... E 20.2 ..... Examination of these results shows that there is no significant difference between any of these shampoos, in regard to the magnitude of the static charge produced on combing the dry hair after treatment with the shampoo. The effect of charge-reducing materials Four samples of hair were given the standard shampoo treatment, and two of them were rinsed in a solution of a cationic detergent. They were then thoroughly rinsed in warm tap water and dried. The charges were determined at a relative humidity of 40 per cent, and a reduction of 95 per cent of the charge was found in both cases.