208 Mr. B. Davies on a new form of Amperemeter 



constant value throughout its entire length (except in the 

 immediate neighboui-hood of the two ends owing to the effect 

 of prominences). This is the case even for the magnetic 

 circuits of figs. 1 and 2, where the induction is rather low, 

 viz. (about) 400 per sq. cm. in the gap. With a magnetic 

 flux of 400 the reluctance of the air is about 300 times that 



Fi<?. 4. 



The central polepiece to which is attached the brass frame supporting 

 the coil, pointer, and spiral springs. 



of the iron ; so that the lines distribute themselves pretty- 

 uniformly on the polar surfaces if the intervening gap is 

 made uniform in thickness. For the higher inductions of 

 800 or 1000 (for which the magnetic circuit of fig. 3 is 

 designed) the magnetic force in the gap becomes still more 

 uniform, for now we have the reluctance of the air 500 times 

 that of iron. 



There is another arrangement of polepieces which 1 would 

 like to describe, although it has not yet been made into an 

 instrument. It furnishes a great deflecting moment, but it 

 has the rather serious disadvantage of being complex in 

 design and — I am afraid — difficult to build. This arrange- 

 ment, like the previous ones, may be attached either to a bar 

 or a horse-shoe magnet, and may be arranged to give with a 

 single moving coil a maximum deflexion of 220°, or it may 

 be designed to give 80° or 90° with a double or twin coil. It 

 may also be designed for small deflexions with a spot of light 

 to be used as a galvanometer, in which case there is no diffi- 

 culty at all in the construction. 



