the Galvanometer used in Electrical Discharges. 173 



with one comb of the induction-machine and the other was free 

 and insulated, so that the galvanometer-wire became saturated 

 with electricity while the machine was at work, there was a ma- 

 terial alteration in the position of equilibrium. But this altera- 

 tion disappeared immediately one of the wires was put in con- 

 nexion with the earth. These preliminary experiments were 

 made partly when the metallic disk on which was the magnetic 

 needle was firmly screwed to the axis of rotation, and partly 

 when this metal disk was removed and instead of it another 

 metal disk of equal size, but without a magnet, was fixed to the 

 axis, in which latter case the system attained its position of equi- 

 librium by a bifilar suspension from two cocoon-threads. As the 

 experiments gave the same result in both cases, the alteration 

 in the position of equilibrium must have been due to some elec- 

 troscopic cause. When the two round disks c and d were re- 

 moved, experiment showed that the changes in the position of 

 equilibrium became considerably greater; hence the disks per- 

 formed their expected service. That the galvanometer with the 

 new arrangement of the moveable parts was not quite unaffected 

 by very strong discharges was doubtless due to the moveable 

 system being somewhat obliquely attracted by the electrical ac- 

 tion ; so that the axis of rotation cannot have hung quite ver- 

 tically. If in this case everything is not accurately centred, so 

 that the centre of gravity lies in the axis of rotation (which is very 

 difficult, if not impossible), it is clear that a change in the posi- 

 tion of equilibrium mutt ensue. Seeing that electroscopic phe- 

 nomena may under certain circumstances so closely resemble 

 magnetic ones that a confusion between them is possible, before 

 a galvanometer is used for actual measurements we must satisfy 

 ourselves that under the present circumstances no electroscopic 

 actions occur. 



II. 

 When a closed conducting-wire is in the vicinity of the circuit 

 of an electrical battery, an electric current is produced in the 

 former when the battery is discharged through the latter. This 

 secondary current in the conducting-wire is stronger the longer 

 the portions of the wires which act upon each other. Hence, in 

 order to obtain strong inductive actions, the wire and the circuit 

 must be coiled spirally near to one another. These currents 

 were discovered almost simultaneously by Henry, Marianini, and 

 Riess. A similar inductive action is also produced if the circuit 

 at one part consists of two branches, one of which is long and 

 coiled as a spiral. In the discharge of the battery, which 

 in this case partially traverses both branches, an induction-cur- 

 rent is formed in the spiral, which discharges itself through the 

 other branch. Baron Wrede has shown from theoretical consi- 



