course, it may
be dried by chemical means and distillation, but this is usually (or
always) unnecessary.

Fig 88.

There is some danger of kerosene containing minute traces of sulphuric
acid, and it and other oils may be conveniently tested for insulation
in the following manner. The quartz electroscope is taken, and the
insulating rod heated in the blow-pipe. The electroscope will now
insulate well enough to show no appreciable collapse of the leaves in
one or two hours' time. Upon the plate of the electroscope is put a
platinum or copper cylinder, and this is filled with kerosene (say) up
to a fixed mark.

The electroscope is placed on a surface plate, or, at all events, on a
sheet of plate glass, and a "scribing block" is placed along side it
and the scriber adjusted to dip into the kerosene to any required
depth. This is done by twisting a bit of wire round the scribing
point and allowing it to project downwards. The point itself serves
to give an idea of the height to which the vessel may be filled. The
liquid is adjusted till its surface is in contact with the end of the
scribing point, and the wire then projects into the liquid and forms
an electrode of constant area of surface. The scribing block is put
to earth. A charge is given to the electroscope, and the time
required for a given degree of collapse of the leaves noted.

The kerosene is then removed and its place taken by vaseline or
paraffin, known to insulate well as a standard for comparison. The
experiment is then repeated, and the time noted for the same degree of
collapse. This test, though of course rough, is generally quite
sufficient for workshop purposes, and is easily applied. Moreover, it
does not require correction for electrometer leakage, as generally
happens when more elaborate appliances are used.

The actual resistance of insulating oils depends so much on the
electrical intensity, on the duration of that intensity, and on the
previous history of the oil as to the direction of the voltage to
which it has been subjected--to say nothing of the effects of traces
of moisture--that quantitative experiments are of no value unless
they are extremely elaborate. I shall therefore only append the
following numbers due to Bouty, Ann. de Chemie et de Physique (6),
vol. xxvii. p. 62, 1892, in which the effect of the conductivity on
the determination of the specific inductive capacity was properly
allowed for:-

Carbon

Bisulphid