354 REPORT — 18GL 



from beloTv. An enlarged section of the block B is given in fig. 3. a, is a 

 ■wooden handle, by \rhich the rod b, Tvith the platinum point d, can be de- 

 pressed so as to come in contact -with the wire of the bridge. When the pres- 

 sui-e of the hand is removed the spring e lifts the handle and breaks the con- 

 tact. The galvanometer wire is screwed in between the metal plates / and g. 

 A pad of gutta percha between the knob h and the handle prevented any- 

 sensible thermal current. To the top of the block was fixed a piece of brass 

 with a slit in it to serve as a pointer. A lens also was fastened to the handle 

 to read fractions of a milhmetre on the scale. The body of the block was of 

 lead, with a slab of ebonite at the top and bottom. The block ran on a tram- 

 way parallel to the scale and wire of the balance. 



A section of one of the mercury cups is given at fig. 2. At the bottom of 

 the cylindrical cup I m n o is placed an amalgamated copper plate, and 

 mercury is poui-ed into the cup ; the plate is held down by the wooden 

 cylinder ^5, and this is kept in its place by the pin r s. This plug fits the 

 cup closely, and is pierced with two or more holes for the terminals to pass 

 through. The cups were propped up with wedges, when placed under the 

 fixed terminals of the balance, that these might press fii-mly against the 

 metal bottoms of the cups. 



Each of the coils R and L had a resistance of about 20 metres of the 

 wire of the instrument. Careful measures were made of the resistance of 

 the wire of the bridge at different points in order to find if there were any 

 very faulty points in it ; this was done by putting the coils R and L in their 

 places, and increasing the resistance of one of them by means of a short 

 piece of wii-e. The effect of this wire was to shift the zero-point. Two 

 coUs, differing about one-tenth per cent., were then placed in the centre of the 

 instrument and the reading taken ; these coils were then reversed and the 

 reading again taken. 



Suppose 2 I the resistance of the circuit from the point B to B' when 

 the short vnve is removed, z the change in the zero-point caused by the in- 

 sertion of the short wire above mentioned, and a: the difference of a pair of 

 readings ; resistances being expressed in millimetres of the wire A A', and 

 lengths expressed in milhmetres of the scale. Then the resistance of a milli- 

 metre of the wire of the instrument about the zero-point is 



a + o X 

 - is the ratio of the two centre coils. 







The value of this expression was found for different points from one end 

 to the other of the wire, and did not vary more than two- or thi'ee-tenths 

 of a milhmetre, an error not considerable enough to aflfect the results obtained 

 ■with the instrument. 



The value of the coil E was thus found. It was placed in the mercury 

 cups m'j, m\, and the cups n\, m^ were joined by a stout copper bar. Two coils, 

 the ratio of the resistance of which was Icnown, were placed in the two centre 

 cups and the reading taken. 



Let - be the ratio of two centre coils, re the reading of scale, which 

 o 



was divided from A' to A, E+r the resistance of the circuit from B' to the 

 point of wii-e opposite that end of the scale nearest to E, viz. A', I the corre- 

 sponding quantity for the other side of the instrument. 



