SPECIFIC INDUCTIVE CAPACITY OE DIELECTRICS. 
583 
The differences of these values show no alteration of specific inductive capacity of 
paraffine due to variations of temperature. 
The absolute value of this condenser with air as the dielectric, obtained by measure- 
ment of it on the sliding condenser described in the Appendix, is 35'394 centimetres; 
and taking the mean of these values with paraffine, 1684 scale-divisions, or 69'552 centi- 
metres, the specific inductive capacity of paraffine is found to be 1*965, that of air being 
taken as unity. The layer of paraffine under the insulated tube had been left in for the 
purpose of supporting it. The correction necessary on account of this increases this 
number to 1*977. 
Appendix. 
The sliding condenser described in this paper was found to be too limited in range, 
and liable to failure of insulation owing to the admission of shreds. Another sliding 
condenser (referred to at pages 576, 578, and 583) was therefore employed of such a con- 
struction as to give greater range and more perfect insulation. The tubes of this instru- 
ment were arranged vertically, and were supported by a wide brass cylinder surrounding 
the insulated tube at a considerable distance from it, and secured to a massive upright 
iron support. To a brass disk forming the cover of this cylinder the insulated tube was 
fastened by a vulcanite collar, into which it was screwed, so as to hang vertically in the 
middle of the surrounding cylinder. This cover also supported the uninsulated tube 
directly above the insulated tube so as to be coaxial with it. The core, instead of sliding 
upon feet as in the instrument formerly described, moved in two V supports cut in the 
brass plates which closed the ends of the uninsulated tube. The points of support and the 
springs which press the moving core against them are formed 
by cutting the brass plates in the manner shown in the 
annexed figure. Another tube, about 7 millimetres greater 
in diameter than the insulated tube, was supported by the 
plate closing the lower end of the wide brass cylinder so as 
to slide vertically round the insulated tube. By altering the 
position of the core and of this outer tube, which was unin- 
sulated, the value of the condenser could be so varied that 
capacities ranging from 47 to 180 centimetres could be 
measured upon it. The position of the outer tube and that 
of the core were indicated upon scales of ^ of (an inch 
engraved upon the tubes themselves. 
a, a. Points of support. 
b. Spring. 
