Form of Resistance-Balance. Ill 



the wood, and is connected by a wire t underneath the base- 

 board with the centre-pin D, and is therefore in metallic con- 

 nexion with the spring x. The battery is connected with the 

 terminals B, B', and the galvanometer with the terminals 

 G, Gr 7 . To the arm H H' is adapted a trigger, T, of such shape 

 that when the button w, which is of ebonite, is pressed down, 

 the spring a, carrying the platinum-iridium knife-edge, is bent 

 inwards until it touches the wire strained round the circum- 

 ference of e. The arm carries a vernier N, which travels round 

 sunk in a shallow groove in the face of the ebonite disk ; and 

 the ebonite is graduated on the face on the margin of the 

 groove. The graduations are cut into the ebonite, and then 

 rubbed over with powdered chalk mixed with gum and water. 

 This gives a graduation very legible and pleasant to look at. 

 The length of the wire is just one thousand divisions ; and the 

 vernier enables these to be divided into tenths. The zero of 

 graduation is so placed that, when the pointer of the vernier 

 reads zero, the knife-edge on the spring x is exactly opposite 

 the extremity of the platinum-iridium wire. 



It is thus clear that the revolving arm carrying its knife- 

 edge can be moved round so that, on pressing the trigger- 

 button w, the knife-edge makes contact at any point of this 

 wire, and thus connects this point with the terminal G'. 



This part of the arrangement answers to the sliding block 

 and piston-contact piece of the ordinary divided-metre bridge. 



3. Method of using the Balance. — Let now two resistance- 

 coils of about equal resistance be provided, and let the coil- 

 terminals of one coil be placed in the mercury-cups p and r, 

 and those of the other be placed in q f and /. And let two more 

 coils be taken of not very unequal resistance which it is de- 

 sired to compare with each other, let the terminals of one be 

 placed in the mercury-cups a and c, and those of the other in 

 V and d ' . It will then be seen that if a battery be connected 

 with B B', and a galvanometer with G GK, that we have the 

 usual Wheatstone's bridge arrangements (see fig. 5 on page 

 113, which gives a diagram of the connexions). Two quart 

 Leclanche cells are best suited for ordinary use. If a more 

 powerful battery is used, there is danger of heating the plati- 

 num-iridium wire, and so expanding it that it may slip down 

 out of its groove. 



The coils in the intervals between the cups p and r and q f 

 and / form two branches ; and the coil in the interval between 

 a and c, together with the resistance of the platinum-iridium 

 wire round to the place where the spring x touches it, forms 

 the third branch, whilst the coil in the interval V d f , toge- 

 ther with the remainder of the wire, forms the fourth. The 



