﻿408 Mr. Rollo Appleyard on Dielectrics. 



rods of Table IX. were all sensitive to these electromagnetic 

 oscillations. The high-resistance rods do not come within the 

 range of measurement of the usual form of Wheatstone's 

 bridge ; the ordinary insulation-test was therefore applied, 

 using a battery of 400 Leclanche cells. In this way resist- 

 ances up to 600,000 megohms could be measured. A rod 

 composed of 4 parts, by weight, of gutta-percha, to 7 parts of 

 brass filings had practically infinite resistance. Another, 

 having 2 parts, by weight, of gutta-percha to 3 of brass, had 

 100,000 megohms. A third, made up in the ratio of 4 parts 

 gutta-percha to 7 of brass, was of infinite resistance. Four 

 rods were made in the proportion 1 gutta-percha to 2 of brass, 

 which is near the critical ratio of conductor to insulator ; 

 their resistances were, respectively, oo , 40 megohms, and 12 

 ohms. The '17 ohm rod exhibited a decided, but not very 

 great diminution of resistance under the influence of the oscil- 

 lator. Apparently, the high-resistance rods are unaffected by 

 the discharges. The 40 megohm rod was not of very con- 

 stant resistance, the spot moved up and down the scale ; the 

 spark had therefore to be passed at moments when the spot 

 halted, — the result was not very satisfactory. At one time it 

 was thought actually to increase the resistance of this rod. 

 The 12 ohm rod was especially interesting from its extreme 

 sensitiveness ; in one condition it had a resistance of 19 ohms, 

 the sudden passage of a spark at the oscillator reduced this 

 by 45 per cent. The rod was generally unstable. The 40 

 megohm rod seemed the one most nearly corresponding, in its 

 galvanometer-readings, with a faulty cable ; the readings being- 

 erratic. Its resistance has, at times, been as high as 770 

 megohms. This rod is probably unstable in some opposite 

 direction to the 12 ohm rod ; they were both made in the 

 same proportions and in the same manner. In Table IX. 

 they appear as No. 9 and No. 12 respectively. 



Alternating Voltages. 



Tests were now made upon specimens submitted to alter- 

 nating currents, and I have to thank Mr. George Bousfield 

 for assisting me in this part of the experiments. I took the 

 strip sprinkled with filings, and two rods ; these were first 

 tested for insulation in the ordinary way, with the following 

 results : — 



Dielectric. Resistance. 



Strip = oo . 



No. 6 = 283,000 megohms. 



No. 9 = 47-2 megohms. 



