and Voltmeter with a Long Scale. 207 



shape, A and B D the polepieces. This form of circuit has 

 been adopted mainly in order to increase the induction. The 

 previous forms do not lend themselves to the use of large 

 cross-sections of steel without becoming inconvenient in 

 shape. In this pattern it is evident that any desired quantity 

 of steel may be used. 



On the soft-iron cylinder B is mounted the brass frame 

 carrying the entire moving system. The air-gap is adjusted 

 for uniformity of induction by means of the D portion of the 

 polepiece, the details of which for the sake of simplicity are 

 not given in the figure. 



Considering next the electric circuit. The main parts of 

 this are the moving coil and the spiral springs. The coil is 

 wound on an aluminium rectangle, which serves the double 

 purpose of a frame to stiffen the coil and a circuit of ex- 

 tremely low resistance for damping. 



In the voltmeter the moving coil contains some 100 or 200 

 turns of the finest wire, while the amperemeter coil has some 

 20 turns of a moderately thick wire. 



In series with the voltmeter moving coil there is placed in 

 the base of the instrument a resistance-coil, on the value of 

 which depend the readings of the meter. 



In parallel with the amperemeter-coil is placed a short 

 length of conductor of a similar material to that of the 

 moving coil. Through this shunt flows the main current in 

 all meters except the centiampere and milliarapere ones. 



The current is led into and out of the moving coil by the 

 spiral springs, which also provide the opposing or controlling 

 force. To partially counterpoise the coil it is placed — with 

 respect to the axis — opposite the pointer. The coil and 

 pointer are shown in position in the plan given in fig. 3. 



A coil of this kind, swung on one of its sides, was used 

 long ago by Lord Kelvin in his siphon recorder. It is also 

 used at the present time by Professor Ayrton in the now 

 well-known Astatic Station Voltmeter. 



Fig. 4 (p. 208) gives a better view of coil, springs, and pointer, 

 and the general method of mounting the entire moving system 

 on the iron cylinder. K is the coil, P the pointer, and Z the 

 arm for zero adjustment. There is here a compactness that 

 becomes important in practice, for by loosening or tightening 

 one screw the moving system with its frame can be removed 

 bodily and placed in position again with ease, and without 

 interfering with the magnetic circuit. The method also 

 renders the balancing of the coil an easy matter. 



In all the meters, when the air-gap is made uniform in 

 thickness, the induction density in the gap has practically a 



