e.

Turpentine.

Benzene (C6H6) at 20 deg. C.

Benzene at

60 deg. C.

Specific inductive capacity

2.715

2.314

2.21

2.22

Specific resistance in ohms per cubic centimetre

1.5 x 1013,

1.75 x 1012

1.56 x 1011

7.9 x 1011

[Footnote: Professor J. J. Thomson, and Newall (Phil. Proc. 1886)
consider that carbon bisulphide showed traces of a "residual charge"
effect; hence, until this point is cleared up, we must regard Bouty's
value as corresponding only to a very short, but not indefinitely
short, period of charge. On this point the paper must be consulted.

March 1897--The writer has investigated this point by an independent
method, but found no traces of "residual charge."]

Information as to the specific inductive capacity of a large number of
oils may be found in a paper by Hopkinson, Phil. Proc. 1887, and in a
paper by Quincke in Wiedemann's Annalen, 1883.

Sec. 114. Imperfect Conductors.

Under this heading may be grouped such things as wood, slate, marble,
etc--in fact, materials generally used for switchboard insulation.
An examination of the insulating power of these substances has
recently been made by B. O. Peirce (Electrical Review, 11th January
1895) with quite sufficient accuracy, having in view the impossibility
of being certain beforehand as to the character of any particular
sample. The tests were made by means of holes drilled in slabs of the
material to be examined. These holes were three-eighths of an inch in
diameter, and from five-eighths to three-quarters of an inch deep, and
one inch apart, centre to centre. A voltage of about 15 volts was
employed. The following general results were arrived at:-

(1) Heating in a paraffin bath always increases the resistance of
wood, though only slightly if the wood be hard and dense.

(2) Frequent exhaustion and readmission of air above the surface of
the paraffin always has a good effect in increasing the resistance of
wood.

(3) When wood is once dry, impregnating it with paraffin tends to keep
it dry.

(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot
be entirely waterproofed in this way.

(5) The resistance of wood with stream lines along the grain is twenty
to fifty per cent lower than when the stream lines cross the grain.

(6) The "contact" resistance between slabs of wood pressed together is
always very high.

The following table will sufficiently illustrate the results obtained.