486 
should be thickly coated with shellac varnish which, after drying, should 
be baked on by heating until it begins to smoke. Mercury cups made in 
this way have been in use in our laboratory for twenty years and are still 
doing good service. 
The figure practically explains itself. By means of the links, L and 
L prime, the apparatus can be connected with any calibrated bridge wire, 
thus adapting it to the Carey Foster method. By changing the thick 
connectors, GC and © prime, to the dotted positions it will be seen, by 
tracing the connections, that the resistance in the gaps, R and R prime, 
are interchanged to opposite ends of the bridge wire. These gaps can be 
adjusted to any width by means of the slotted support shown in the 
figure. A piece of mica, M, is used to separate the strips at the point of 
crossing. 
As the conductors are so arranged that the arms of the bridge are at 
all times symmetrical, the dimensions of the apparatus are immaterial. 
However, a scale of centimeters is added as a suggestion of size for those 
who may care to construct apparatus of the size figured. It might be 
added, that apparatus similar to the above has been used at the DePauw 
physics laboratory for a number of years and has proved very satisfactory 
for student work. 
