634 ALEXANDER MACFARLANE ON THE 
with the metallic parts of the air-pump, and by means of a chain with the 
uninsulated conductor of the Holtz machine. Brass balls and discs of various 
sizes were made to order, capable of being screwed on to the ends of the rods. 
On the table, and at a distance of about six feet from the receiver, was a stand 
supporting two insulated brass balls, the one fixed, the other having one degree 
of freedom, viz., of moving in a straight line in the plane of the table. The 
fixed insulated ball A was made one conductor with the insulated conductor of 
the Holtz and the rod of the receiver, by means of a copper wire insulated 
with gutta percha, having one end stuck firmly into a hole in the collar of the 
receiver, and having the other fitted in between the glass stem and the hollow 
in the ball, by which it fitted on to the stem tightly. A thin wire similarly fitted 
in between the ball B and its insulating stem connected the ball with the insu-_ 
lated half ring of a divided ring reflecting electrometer. 
Thus there were in all four conducting systems,—Frst, the interior surface 
of the room, with the uninsulated conductor of the Holtz, the lower electrode - 
inside the receiver, the outside case and the uninsulated half of the divided 
ring of the electrometer. Second, the insulated conductor of the Holtz, with 
the rod of the receiver, the upper electrode, the covered wire, and fixed 
insulated ball. Third, the movable insulated ball, with the wire and insulated 
half of the divided ring of the electrometer. Fourth, the Leyden jar of the 
electrometer, with the vertical wire and horizontal needle. To keep these four 
systems of conductors insulated from one another was a condition essential to 
accuracy in the experiments. To satisfy this condition the two glass insulating 
stems A and B were thoroughly heated each morning before the fire, the upper 
part of the receiver was coated with shellac, while the lower part was coated 
in the inside with tinfoil, connected by a strip with the lower rod. The fourth 
system was generally charged before taking a series of observations by communi- 
cating to it a half-inch spark from the electrophorus. Its insulation was several 
times tested by taking two observations, distant by the lapse of twenty-four 
hours, of the reading at discharge under conditions constant, excepting as to 
the charge of the moving needle. I find that the readings on the 30th and 31st 
May for a spark ‘5 centimetre long, and under given conditions, were 99 and 
79°6 respectively ; from which we may infer that four-fifths of the chart 
remained after a lapse of twenty-four hours. 
When the potential of the second system was raised by driving the machine, 
the potential of the third was also raised ; and this went on until a discharge 
took place between the electrodes inside the receiver, so that the deflection of 
the spot of light from zero was an indication of the difference of potential of 
the second and first systems when the discharge took place. By breaking 
the contact between the conductors of the Holtz machine before beginning to 
turn the wheel, and by turning the wheel slowly and steadily, we made the 
image of the wire move continuously, and to be at rest at the instant of dis- 

