850 PROCEEDINGS OF THE AMERICAN ACADEMY 



second four corresponding holes in the ebonite, a' ,c' ,h' , d' . When 

 these cylinders are in the holes they act as steady-pins. In the part 

 a c b d are eight parallel channels, each about | mm. wide and 1 mm. 

 deep, symmetrically disposed across the square. The seven ridges be- 

 tween these channels are each rather narrower than the channels, so 

 that the whole width from outside to outside is a little less than 8 ram. 

 On the part of a' 1/ c' d' corresponding are seven similar channels, the 

 centre one being in the centre of the square. On each side of the 

 square m. ah c d are four similar channels, and on each side of that in 

 a' b' c' d' are three channels. When the two disks are put together 

 (one may be supposed to revolve like the lid of a box about the imagi- 

 nary hinge X T), the ridges on one fall into the channels of the other. 

 At e ,y , g ,h , are split pins of platinum, and at e' ,f' , g' , h' , corre- 

 sponding hollow cylinders of platinum to receive them. 



The current enters by the brass of the hollow steady-pin, a, and 

 passqi eight times across the square as shown, then crosses over through 

 the pin e to c' , passes seven times across the square, back by A' A to the 

 first disk and then out by the pin h. Where the current crosses the 

 square, it is conducted by strips of iron, each a little less than 0.5 

 mm. wide and about 0.004 mm. thick, laid in the channels. There 

 are therefore, when the disks are put together, fifteen of these strips 

 drawn parallel across the square, each at the bottom of its channel, of 

 which eight lie in one plane in the first disk and seven in another plane 

 in the second disk, the two planes being about 1 mm. apart when the 

 disks are fitted together. The strips in each set lie ojjposite the open- 

 ings between the strips of the other set. The other current enters at 

 c, goes along four iron strips, up the pin f tof , then three times 

 along iron strips, then round to the other side of disk number two, then 

 three times along iron strips, then down a copper wire to g' , then 

 through g back to disk member one, then four times along iron strips 

 and out at d. There are therefore twenty-nine strips disposed in two 

 circuits,* fifteen in the central circuit (these fifteen form virtually one 

 arm of the bridge), and fourteen in another circuit which surrounds the 

 former. These fourteen, disposed in two groups for symmetry, form 

 then the second arms. Every joint is soldered. 



* These strips are preferably not soldered, but continuous.beins struck by a 

 punch from a single film of metal. Tlie number of tliese and their size is varied 

 in different instruments, wliicii may be adapted to a low or a high resistance 

 galvanometer. Tlie slight difference of resistance between the central and side 

 strips it is intended to make up by resistances introduced within the balance- 

 case, so that the use of a resistance box will not be necessary. 



