SURFACE LEAKAGE O F PYREX GLASS BY W . A . YAGER B X D S. 0 . MORGAN
Introduction The direct current surface leakage of various insulating materials has been studied a t a number of humidities by Curtis,’ but there has been very little study of the alternating current surface leakage. The importance of the problem from the standpoint of telephone insulators, especially those used for the transmission of carrier frequencies has been pointed out by L. T. \\-ilson.2 A study was undertaken to determine the effect of relative humidity, frequency, and temperature upon the total conductance, total capacity, surface conductivity, surface capacity, and power factor of various kinds of glasses. This paper gives the results obtained on E. J. Pyrex glass a t s z 5 O C . and p ° C . over the range of humidities from o to 96% and at frequencies from I t o IOO K. C. A A study is also being made of the effect of composition of the glass upon surface properties and upon the weathering. Test Condenser Considerable difficulty was experienced in preparing a satisfactory test condenser, i.e., one which would have a capacitance and conductance which would be easily measurable with the apparatus available. It was necessary in order to measure with sufficient accuracy the lowest conductance to be studied, namely, that a t 0% humidity and I K.C., to have electrodes 100 cm. long separated by a 0 . 2 5 mm. gap. A test condenser of the proper characteristics was obtained by making two adjacent metallic spirals I / Z ” wide on a tube 1-318’’ in diameter and 8” long. This test condenser is shown in Fig. I . The metal electrodes were put on the glass by silvering, the gaps between the electrodes being produced by keeping a fine rubber band wound on the tube during FIG.I the silvering process. In order to make the coating Pyrex Test Condenser for studying Surface Leakage adhere better, the glass was first sand blasted, the gap being protected by a large rubber band. After silvering, the electrodes were copper-plated and finally gold-plated to reduce the resistance and to prevent tarnishing. Contact with the spiral electrodes E 1
2
H. L. Curtifi: E. S. Bureau of Standards, Scientific Paper S o . 234. L. T. Wilson: J. Am. Inst. El. Eng., 49, 1j36-48 (1930).
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was made by platinum wires sealed to the glass as indicated in Fig. I . The short projecting end of the platinum wire insures a good contact with the electrode regardless of the bending of the other end of the wire which occurs in handling. The inner walls of the tube were cleaned and the space sealed off a t A under a vacuum to prevent any conduction over this surface. The exhaust vapors passed out of the apparatus through the inner sealed tube T which also served to support the test condenser in a hard rubber stopper S which fitted into the humidity chamber. Humidity Apparatus Efficient humidity control was obtained by a dynamic method in which dry air and saturated air were mixed in various proportions to give any desired humidity. A description of this method may be found in a paper by Walker and Ernst.’ The humid air thus produced enters near the top of the humidity chamber, is forced down past the test condenser and escapes t o the atmosphere through the inner supporting tube T. A small capillary was inserted in the outlet to prevent air from diffusing back into the chamber. The highest humidity obtainable with this apparatus was 96%. The humidity chamber and saturator were maintained at constant temperature in a thermostat and the surrounding temperature was kept slightly higher to prevent condensation of moisture in the “wet” line. General Considerations Measurements of total conductance and total capacitance were made at zj°C and 50°C + .oso a t I , 3, I O , 3 0 , j o , 7 5 , IOO K.C. and at a number of humidities between o and 96y0 relative humidity. These quantities mere measured as equivalent parallel conductance and capacitance on a capacitance and conductance bridge, described by Shackelton and Ferguson.2 The conductance could be determined to IO-^ mhos. above 30 K.C., and to 1 0 - l ~mhos. below this frequency. The capacitance could be measured accurately to 0.1mmf. In this paper, G and C are used to indicate conductance and capacitance respectively. The subscripts V, S, and T, are used to denote volume, surface, and total properties respectively. Curtis studied the effect of humidity upon the volume resistivity of various substances. In the case of glass, the results on one sample indicated a slight change of volume resistivity with humidity, but it was too small to be certain of. In our tests, the conductance of the pyrex test condenser a t zero humidity before and aft,er a run checked within the error of measurement. The change of volume conductance with relative humidity is therefore insignificant in comparison to the large change in the surface conductance with relative humidity. The surface quantities are therefore obtained by subtracting the volume quantities as measured at zero humidity from the total quantities measured ‘ A . C. Walker and E. J. Ernst Jr.: Ind. Eng. Chem., Anal. Ed., 2, 134 (1930). R. J. Shackelton and J. G. Ferguson: Bell System Tech. J., 7, 70-89 (1928).
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a t any other humidity. I t is apparent that the error involved in determining G, and C,may be rather large at low humidities where the differences GT - G , and CT - C, are small. The surface conductivity is determined by dividing the surface conductance by a constant which is the sum of the ratios of length t o width of the two gaps. The values for this constant were 439 j at 2 j"C, and 41 j j at j o " C, the difference being due to the removal of j . 4 cm. of one electrode between puns. The conductance and capacitance a t j o o C are consequently about 6% lower than they would otherwise have been. The surface conductivities, y , are, however, directly comparable. .70 .68
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