Mr. A. Schuster's Electrical Notes. 9 



trometer, or revolved and passed through the electro- 

 meter as if it were a zigzag gateway,, without cutting the 

 metallic part of the circuit anywhere. Jf the electrometer is 

 removed and a proof-plate is used, by touching one terminal 

 with it and measuring the work done when it moves to the 

 other, the result is the same. If the lines are stationary, the 

 proof-plate is charged by one terminal and repelled by it and 

 attracted by the other. If the lines rotate, the proof-plate is 

 urged forward by the lines of induction cutting it at right 

 angles to its path. The reading is the same in both cases. 

 The lines of induction are a mere convention, and there is no 

 way of finding the seat of the electromotive force. All that 

 can be said is that it is cyclic, and that the difference of 

 potential of any two parts of the circuit of the same metal can 

 be measured. In some cases the seat where power is spent 

 can be determined, and in others it cannot. 



Similarly in the cell, the seat of expenditure of power can 

 sometimes be told, as when there is a resistance in circuit; 

 sometimes it cannot, as when the cell works a unipolar, or, by 

 extension, any other motor. The difference of potential be- 

 tween any two points of the same material can be measured, 

 but all that can be said is that the electromotive force of the 

 whole circuit is cyclic. 



II. Electrical Notes. 

 By Arthur Schuster, F.R.S* 



1. The Vector Potential. 



THERE is a general tendency at present among elec- 

 tricians to dispense as much as possible with the " Vector 

 Potential," but I believe that many of the difficulties which 

 have led to the restriction of its use may be removed. At 

 any rate there seems room for the following renewed dis- 

 cussion of some well-known equations. 



I was led to the investigation through an attempt to find 

 the magnetic effects of currents distributed through space in 

 such a way that the lines of flow should be identical in form 

 to the lines of induction in certain simple magnetic systems. 

 We might, for instance, wish to find the components of mag- 

 netic force which are produced by currents in an unlimited 

 homogeneous conductor, the electrical potential at every 

 point being the same as the magnetic potential due to a simple 

 magnetic shell. The case could be realized by a conductor in 

 * Communicated by the Author. 



