t 
573 
XX. Measurements of Specific Inductive Capacity of Dielectrics, in the Physical Labo- 
ratory of the University of Glasgow. By John C. Gibson, M.A., and Thomas 
Barclay, M.A. Communicated by Sir William Thomson, F.B.S. 
Received November 23, 1870, — Read February 2, 1871. 
The object of this paper is to describe the instruments and processes employed in a series 
of experiments on the specific inductive capacity of paraffine, and the effect upon it of 
variations of temperature. 
The chief instruments employed in this investigation were the Quadrant Electrometer, 
and two others which will now be described. 
I. The Platymeter, so named by suggestion of the late Dr. Whewell from its use in 
the comparison of electrostatic capacities, was, in a rudimentary form, shown to the 
Mathematical and Physical Section of the British Association at its Glasgow Meeting 
in 1855, by Sir William Thomson. It consists of two condensers of equal capacity, each 
of which has one of its opposed surfaces in metallic connexion with the corresponding 
surface in the other. 
In the instrument employed (see Plate XLIII. fig. 1) the conductor constituting these 
two connected surfaces is a brass cylinder (c c ) 22-94 centimetres in length and 5T 
centimetres in diameter, supported at its ends in a horizontal position by two vulcanite 
stems ( d d, d d). Round it are placed two equal and similar pieces of brass tubing 
(pp,p'p'), 7’68 centimetres in length and 8 - 6 centimetres in diameter, each supported 
by a vulcanite stem ( e e , e e), so as to be concentric with the cylinder and at equal distances 
from its ends. The four supports ( d d, d d, ee, e e) are fixed in a massive plate of iron 
(i i ) ; and the whole is enclosed in a stout metal box ( m m, m m), an electrode (n n) from 
the inner cylinder being carried through an insulating plug (o) of paraffine in the end 
of the box, and another (q q, qq) from each of the tubes through similar plugs (rr) in 
the top of it. By this means shreds and dust are excluded, and paraffine is found to be 
so good an insulator that no inconvenience is experienced by employing it in this way. 
II. The Sliding Condenser is a condenser the capacity of which may be varied by- 
altering the effective area of the opposed surfaces. The following form of this instru- 
ment was employed (see fig. 2). Two pieces of brass tubing (a a, bb), 2-48 centimetres 
in diameter and 26-58 and 35*3 centimetres in length respectively, are supported end to 
end upon vulcanite stems (cc, d d) securely fixed in a massive iron plate (h h ). Within 
these tubes, and coaxially with them, moves a brass cylinder ( e e) 36-6 centimetres in 
length and 1-15 centimetre in diameter, supported at one end upon four brass feet (f,f) 
which slide on the inner surface of the longer tube, and loaded at that end so as to rest 
4 k 2 
