686 ALEXANDER MACFARLANE AND P. M. PLAYFAIR ON THE 

We noted that after the conductors were fully discharged and the image 
had ceased to oscillate, it made an excursion in the negative direction, to the — 
distance of ten divisions or so in the interval between the pairs of observations. 
These observations are plotted on diagram 4. The ordinates of the cooling 
curve are less than the corresponding ordinates of the heating curve—a result 
confirmed by two other series of observations. The falling off cannot be 
entirely due to a leaking of the charge of the electrometer ; for readings taken 
after twenty-four hours showed that not more than one-third of the difference 
could be so accounted for. The bulb of the thermometer, being in the place 
indicated by fig. 2, must in each case have given the temperature of the air 
between the discs ; hence the diminution was probably due to the temperature 
or other state of the discs. 
Several series of observations were undertaken to determine whether the 
electrometer, by means of which all these observations were made, gives 
deflections which are strictly proportional to the inducing charge. A special 
method was required on account of the want of sensitiveness of the electro- 
meter, and the considerable range of scale used. Acting upon Professor Tart’s 
suggestion, we put a charge upon the inducing ball of the pair on the stand, 
observed the deflection on the scale, then divided the charge by putting a 
similar and equal ball into contact with the ball on the stand, and then removing © 
the auxiliary ball, read again, and so on. We obtained observations, all of 
which agree with the following :— 
TasLeE VI.—Calibration of Electrometer, 28th June 1878. 




Zero. Deflection. Difference. 
1 » | nN. | n—n. 
ae: = = ashe i | 
Before division, . . . 505 165 340 
After 1st division, . . 1s 345 160 
After 2d division, . . i 430 15 
After 3d division, . . i 473 32 
After 4th division, . . aS 493 | 12 
After 5th division, . . 43 503 2 
After 6th division, . . ey: 509 | —4 







When the inducing ball was connected with the ground the zero was 515; 
when the electrometer was connected with the ground, 505. 
When the entries of column fourth are plotted as ordinates to the numbers” 
256, 128, 64, 32, 16, 8, 4, as abscissz (see diagram 5), they are seen to indicate 
a straight line, the intercept of which on the axis of ordinates is slightly 
negative, a peculiarity borne out by all our other series of observations. The 
