402 Dr. A. Macfarlane on the Disruptive 



Measurement of the Difference of Potential required to pass a 

 Discharge through Air at the Atmospheric Pressure between 

 two Balls , between a Ball and a Plate , and between a Point 

 and a Plate. 



In the first case the electrodes were two spherical balls, 

 each 1*5 centim. in diameter; and they were attached in the 

 same manner as the disks. The curve obtained (diag. 6) coin- 

 cides at the beginning with a parabola, but is afterwards bent 

 in towards the axis of abscissa?. It is quite similar to that 

 obtained by Mascart under the same conditions*. When the 

 distance was 8 centims., violet sparks began to precede the 

 white spark, and the electrometer simultaneously began to 

 indicate the occurrence of a small discharge before the com- 

 plete discharge. This caused the reading to be somewhat 

 more ambiguous. At 10 centims. the electricity began to 

 escape from the insulated wire. 



On the following day a series of readings for the second 

 pair of electrodes was obtained, and without any change in 

 the arrangement other than the substitution for the lower ball 

 of a circular plate 16'2 centims. in diameter. The plate was 

 uninsulated, and the upper ball charged positively. The curve 

 is drawn on the same diagram with that for the two balls; it 

 also coincides with a parabola at the beginning, but at the 

 reater distances is still more bent towards the axis of abscissae, 

 iolet sparks (giving an incomplete discharge) began much 

 sooner, at 3*5 centims. compared with 8 centims. 



A series of readings for the third pair of electrodes was ob- 

 tained on the third day. A conical brass point was substi- 

 tuted for the upper ball. With the increased capacity due to 

 the couple of Ley den jars the readings were regular; without 

 it they were irregular. Up to 5 millims. the discharge was 

 effected by a spark ; but for greater distances there was only 

 a faint brush or glow at the point, accompanied by a partial 

 discharge. 



Comparison of the Positive and Negative Electric Discharge. 

 To complete the preceding investigation, it is necessary in 

 the second and third cases to take readings for several distances, 

 with the ball or the point charged first with the one and then 

 with the other kind of electricity. This I have been able to 

 do ; but as it was in the United College, St. Andrews, the 

 apparatus was somewhat different. Instead of the receiver 

 arrangement, I had a Henley's discharger, using respectively 

 for charged electrode the conical point of one of the rods and 

 * Mascart's Electricite, t. ii. § 479. 



