574 
MESSRS. GIBSON AND BARCLAY ON MEASUREMENTS OE 
stably upon them. Attached to this cylinder is a vertical arm (g) which projects through 
a slot extending along the upper part of the surrounding tube. This arm carries an 
index (i) by which the position of the sliding cylinder or core is indicated on an attached 
scale (kk). This scale is 22-86 centimetres long, and is divided into 360 divisions, each 
of an inch in length. The tube (b b) which supports these parts of the instrument, 
though placed on vulcanite stems for convenience in testing, is in working kept in con- 
nexion with the earth. The other (« a), which may therefore be called the insulated 
tube, is closely surrounded by a sheet of metal (l l), which is securely fastened to the 
iron base of the instrument. This sheet, being connected with the earth, guards the 
insulated tube against electric disturbance, and adds largely to the capacity of the 
condenser. As an additional security against disturbance, another sheet of metal (m m) 
is fastened round the former at some distance from it. 
In using these instruments the following connexions are made (see Plate XLXV. fig. 3). 
The inner cylinder or core ( c c ) of the platymeter is connected with two ( q , q") of the 
quadrants of the electrometer, the other two (q', q'") being connected with the case of 
the electrometer ( n n n ) and with the earth. The condenser to be measured is connected 
with one of the tubes or sides of the platymeter, and the insulated tube of the sliding 
condenser with the other. All other parts of the sliding condenser and the box and 
base-plate of the platymeter are kept in metallic communication with one another, and 
with the disinsulated pair of quadrants of the electrometer. 
Now suppose the two sides (p andjf) of the platymeter to be exactly equal, and the 
condensers to be compared, say A and B, also equal. Let the insulated quadrants ( q , q") 
be temporarily connected with the disinsulated pair (q', q 1 "), and let the condenser A, 
together with p, the side of the platymeter with which it is connected, receive a positive 
charge. Then c, the core of the platymeter, becomes by induction negatively electrified. 
The earth-connexion of the quadrants ( q , q") is now broken, and the charge thus insulated 
upon c remains masked by the action of p so as to cause no deflection of the needle. A 
connexion is now established between^? andj/, the two sides of the platymeter, so that 
on the supposition of equality already made, the charge is now distributed over a system 
of double capacity, and the potential of the whole becomes half of that of A and p before 
distribution. Both sides of the platymeter are now acting upon c. each with half the 
original intensity of p , so that the resulting effect upon the core being unaltered, no 
deflection of the needle takes place. 
If the condenser A be of greater capacity than the condenser B, then, on connecting 
p andy, the capacity of the whole charged system is less than double that of A and^?, 
and its potential, therefore, greater than half the original potential of A and p. Each 
side of the platymeter, therefore, is now acting with more than half the original intensity 
of p. The result is an increased action on the core of the platymeter, and the consequent 
liberation of positive electricity to act on the needle, which, in the ordinary arrangement 
of the electrometer, is deflected to the right. 
If, on the other hand, the capacity of the condenser A be less than that of the con- 
