ELECTROMAGNETIC UNIT OF ELECTRICITY TO THE ELECTROSTATIC UNIT. 593 
Vo = kh 
c = 
1 
I 
y neglecting 
c'lTthll __ p — 2irliil 
J 
Thus <T, the surface density on the plane of xz, 
' 0 
Airh 
1 — 
. 27r.r 27r'] 
/3.sm —- 
P^TThjl _ 
J.7r 
Vq J , _ (?/ — 
47r/i 
p27r/i/i _ f.-lirhlL 
Thus, if we choose h so that it is the mean distance between tlie plates, for the 
breadth on which we wish to find the charge, the second term will vanish in our 
integration, and we get for Q the quantity of electricity on a breadth x 
Thus we can use the ordinary formula even when the plates are slightly Inclined, 
provided h is the mean distance. Any correction to this will be the order of the 
square of the inclination at least, and in our case may be neglected. 
Measurement of Dimensions of Condenser. 
The dimensions are all referred to the standard metre of tlie Cavendish Laboratory 
which has been compared with the standard of the Board of Trade. The errors of 
the divisions are too small to affect the measurements given below. The comparison 
of the lengths with the standard metre was made by means of a pair of reading 
microscopes with micrometer screws. The pitch of the screws is accurately 5 ^-th of 
an inch, and the head of the screw is divided into 100 parts, so that one division of 
the screw-head corresponds to '0002 inch. The tenths of divisions are easily read 
iind are recorded. The screws were tested by Mr. Fitzpatrick when working with 
Mr. Glazebrook at the Specific Besistance of Mercury, and were found to be free 
from sensible error in either pitch or uniformity. 
The standard metre is correct at 0 ° C., and its temperature coefficient is '000017 
per 1 ° C. 
We require the dimensions of the condenser at 16° C. The metal of which the 
condenser is made is much the same as that of the standard metre, so that if we 
assume that the temperatures of the condenser and standard metre are the same at 
mdcccxc.—A. 4 G 
