4895 
A SrmmpiLe Form oF THE CAREY Foster BRIDGE. 
J. P. NAYIOR. 
Probably there is no better method than the Carey Foster method for 
comparing nearly equal resistances or for measuring small changes in re- 
sistances such, for instance, as those due to change of temperature. While 
many arrangements have been devised for interchanging the resistances, 
as is necessary in this method, the following arrangement is believed to be 
sufficiently novel to make it worth while to present it to the Academy and 
it is also thought to possess some advantages over other forms of the 
apparatus. 
Its advantages are that it is readily adapted to any width of resistance 
terminals; the arrangement of connections and relative position of the 
resistances is such that they can be seen at a glance; and the apparatus 
is so simple in construction that it can easily be made by anyone who 
can solder a little and can drill a few screw-holes in the straight copper 
strips of which it is made up. The whole apparatus can be built up on 
well varnished hardwood as a base, the copper strips being fastened to 
the wood base with wood screws. To help the insulation, it has been 
found to be a good plan, after the screws have been screwed home, to re- 
move them, one at a time, put a drop or two of thick shellac in the hole, 
and then put the screws back again. 
No. 10 or 12 gauge copper strip, two centimeters wide, can be used 
for the conductors. In the figure, the double, concentric circles represent 
binding posts and the single circles mercury cups. The latter are made 
from .41 copper rim-fire cartridge shells. 
It may not be generally known that these shells, although quite thin, 
make very satisfactory mercury cups if soldered directly on top of such 
copper strips as here mentioned. If blank shells cannot be obtained, the 
charges may be drawn from the loaded shells. The charges should not be 
shot out, for the firing pin will nick the shells and cause them to leak. 
In order to prevent any mercury from coming in contact with the solder 
and loosening the shells, they and the strips, after soldering is done, 
