THE MEASUREMENT OF CURRENT 55 



soft iron parts, for this causes the indications of the direct- 

 current instruments to depend on their previous history. 



9. Pivot friction must be reduced to a minimum and the mov- 

 ing system properly balanced. 



10. The instrument must be dead beat, that is, critically 

 damped, in order that fluctuations of the load may be followed 

 with certainty and that the time necessary for taking readings 

 may be reduced to a minimum. 



11. The graduation should be convenient. This reduces the 

 liability to mistakes in readings which have to be taken hurriedly. 



Moving-coil Ammeters. In direct-current instruments, the 

 fulfilment of the conditions stated above is most readily obtained 

 by employing the moving-coil principle. 



The first thoroughly practical instrument of this class was 

 designed by Edward Weston in 1888. It will be described, in 

 its present form, as a typical example of a moving-coil ammeter. 



Weston Standard Portable Ammeter. This instrument is 

 essentially a shunted D'Arsonval galvanometer, so designed that 

 it fulfils the requirements of portability and general reliability. 



The magnet, which is of the horseshoe type, is made of tungsten 

 steel and is artificially aged; the cross-section is about 1.25 by 

 0.3 in. Carefully shaped soft-iron pole pieces are attached to 

 the magnet by screws so that the space between them is cylin- 

 drical. In this space is placed a soft iron cylinder supported 

 from a brass yoke attached to the pole pieces. The air gap is 

 about 0.04 in. wide; consequently, the coil moves in a radial 

 field (see page 35). The movable coil, of copper, is wound on 

 an aluminum frame which also serves as a damping device to 

 make the instrument dead beat. The movable system is pro- 

 vided with steel pivots which turn in two jewelled (sapphire) 

 bearings which are carried by non-magnetic yokes attached to 

 the pole pieces in such a manner that the coil is truly centered. 

 The directive force is given by two flat spiral springs, one above 

 and one below the coil; they are made of non-magnetic material 

 and also serve as leads to the movable coil. The inner ends of 

 the springs are attached to brass collars which form the terminals 

 of the coil, the outer ends to the extremities of two insulated 

 crossarms which can be moved coaxially with the coil, to adjust 

 the zero. In the more recent instruments this adjustment may 



