Altevnating Currents to Cah'hratio)i of CapacUij-boxes. 709 



:i]>j)aratiis of oreat utility {V\ where there is no difFereiice \\\ 

 })hase between the ciu'rents in the two arms of the bridge. 

 and (2) where the phases of the current in the moving coil 

 and of the magnetic field are far removed from quadrature. 



It is very important in connexion with the use of this 

 instrument that the ends of the movable coil should be per- 

 manentlv attached to the bridoe — to use Wheatstone brido;e 

 terminology the key must be in the battery-circuit and not 

 in the galvanometer circuit, in fact it is absolutely essential 

 that there should be no key in the galvanometer circuit. 

 The reason for this is as follows : — The " false-zero '' position 

 of the movable coil, i, e. the position it takes up when the 

 alternating current is magnetizing the iield, is a function of 

 several things, (1) its zero position for no alternating iield, 

 (2) the intensity of the alternating field, (3) the shape, size, 

 and conductivity of the metal frame on which the coil is 

 wound, (4) the resistance and capacity in circuit as a shunt 

 on the movable coil. Thus, for example, suppose the field is 

 magnetized by an alternating current and that we join up 

 the terminals of the movable coil to a resistance-box, we 

 shall probably find that the " false-zero " position depends 

 on the resistance ; we shall certainly find that this is the case 

 if wx twist the head, which supports the strip carrying the 

 coil, so that the latter is in an asymmetrical position in the 

 field. If the coil be short-circuited we find that it takes up 

 a position of great stability in what would be magnetically 

 a symmetrical position were it not for the torsional effect of 

 the strip ; as it is, the position taken up is intermediate 

 between its original zero position before the magnetization 

 of the field and the position it would take up if there w^ere 

 no torsional rigidity in the strip. This being so it is clear 

 that any increase in resistance in the short-circuiting box 

 will have the effect of diminishing the deflexion due to the 

 alternating field, and so the " false-zero ^' position of the coil 

 is a function of the resistances w^hen that coil is used as a 

 galvanometer in an ordinary Wheatstone^s bridge circuit. 

 The apparent vagaries due to this " false-zero ^' bothered us 

 a good deal at one time, although no doubt we ought to have 

 foreseen all these points. 



A number of experiments were made upon the positions 

 of equilibrium of copper frames of various shapes and sizes 

 when suspended in the alternating field, and although there 

 is nothing new in any of these results (all of them being 

 entirely in conformity with Elihu Thomson's experiments on 

 the repulsion produced by alternating currents), vet it may 

 be well to rei)roduce some of them so that if the instrument 



