(561 ) 
XVII.—On the Disruptive Discharge of Electricity. Part IV. (Plate XIV.) 
By A. Macrartang, M.A., D.Sc., F.R.S.E., and P. M. Prayratr, M.A. 
(Read 6th January 1879 and 19th July 1880). 
The following experiments, in continuation of our research on the 
discharge of electricity, were made in Professor Tart’s Laboratory during the 
College Session 1878-79, and papers briefly describing the results were read 
before the Society on 6th January 1879 and 19th July 1880. 
Discharge of Electricity through a Paraffin in different States. 
By favour of Mr CaLtpEerwoop, Manager of Addiewell Chemical Works, 
we obtained a quantity of a pure paraffin of low melting point. We have 
recorded in Table I. a series of observations which we made to compare its 
electric strength when solid and when liquid with that of air at the 
atmospheric pressure. By the electric strength of a dielectric we mean the 
ratio of the electromotive force required to pass a discharge through the 
dielectric to that required for air under the same conditions, and when the 
length of spark is 5 mm. It is necessary to make the comparison at a fixed 
length of spark, as in the case of the gases the electromotive force is not 
proportional to the length of the spark. In the case of some liquids, we 
found that we could not take measurements for a 5 mm. spark; in such 
cases we make the comparison at the greatest length of spark for which 
observations have been taken. 
A cylindrical glass dish 7 inches in diameter, had a brass screw passed 
through the centre of the bottom, to which one of the 4-inch circular plates 
could be screwed on, as represented in the woodcut. 
The lower plate was connected with the earth by 
attaching a wire to the outside part of the brass 
screw. The upper plate was suspended from the rod 
of an air-pump receiver so as to be parallel to the 
lower plate, and at a distance of 3mm. A sufficient 
quantity of the liquefied paraffin was poured in to fill 
the dish to the height indicated in the woodcut, and it was allowed to solidify 
for twenty-four hours. We were particularly careful to note properly the 
reading for the first spark, as we knew that we could not get a second of 
VOL. XXIX. PART II. 7c 
