ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 113 
flows through B, P B, (Fig. 7), a large resistance of amount R,+R,. One 
plate of each condenser is in contact with B, and B, respectively; let vj, 
VY, be the potentials at these points. The other plates A,, A, are insulated 
and connected together and to the galvanometer G; the other pole of the 
galvanometer can be connected to p through the insulated key k,. The 
galvanometer can be replaced by an electrometer. Let R, be the resistance 
P B,; Ry the resistance P B,. Suppose the point pbe put to earth, the rest 
of the circuit being insulated ; then if c, Cc, be the capacities, it is easy to 
see that there will be no current through the galvanometer on making the 
key K), if Cy R, =Co Ro. 
Now, in the case of a mica or paraffin condenser the capacity is a 
function of the immediate past history of the condenser, and different 
values will be found for the resistances R,, Ro, according to the time the 
charging has lasted. Dr. Muirhead, however, who uses the method 
largely, has shown how to obtain the instantaneous capacity from the 
observations. His method is described in the following extract from a 
letter to myself.! In the method as described one pole of the battery is 
to earth instead of the point Pp of Fig. 7. 
Dr. Muirhead writes: ‘I have ‘05 microfarad nearly in air condensers, 
and a series of mica condensers of ‘1, *2, 3, °331 (original 1/8), and *498. 
ee 
Fia. 8. 
(original 5) mf. capacity, all enclosed in a double air-tight box, to keep- 
the temperature as uniform as possible. The capacity of these standards. 
is determined periodically by both the tuning-fork method (using a. 
revolving commutator instead of the tuning-fork) and by the ballistic 
galvanometer method. One can make comparisons of these condensers. 
among themselves, and with other condensers by the method I adopt, to an 
accuracy of 4in 10,000. The temperature coefficient of shellacked mica 
condensers is about ‘018 per degree Centigrade, and of paraffined mica 
“034 per cent. 
‘Let s, be the capacity of the air condensers ; 
» So ” = condenser to be compared with air 
condensers. 
* After making battery contact, supposing the charging of the cor- 
_ densers to be instantaneous and the absorption nil, then we have 
v 8,;=(V—v) So 
’ See also Hlectrician, September 5, 1890. 
1890. I 
