485 



A Simple Form of the Carey Foster Bridge. 



J. P. Naylor. 



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, 



