288 Mr. W. H. Preece on some Physical Points 



water, all effects of static induction will cease between 1 and 

 2. In water they are not entirely eliminated, for water is a 

 very poor conductor ; but they are so reduced by its in- 

 fluence, as my experiments between Manchester and Liverpool 

 and between Dublin and Holyhead have shown, that, if the 

 water or wet serving had been a perfect conductor, they would 

 have been removed as far as regards static induction. 



But we have to regard magnetic induction as well. Besides 

 establishing a field of electric force around 2, a current flowing 

 through that wire establishes a magnetic field around it, whose 

 lines of force are circles, and whose directions are at right 

 angles to the lines of electric force. Let us regard that line 

 of force cutting 1. Each time a current commences, and 

 each time it ceases, in wire 2, a line of magnetic force cuts 

 wire 1, and produces in that wire a current of induction in the 

 same direction as that produced by static induction. Now, if 

 we make the screen of iron, those lines of force terminate in 

 the iron and wire 1 is freed. Hence, if we sheath the wire 1 

 with iron, it is not only freed from the effects of static induc- 

 tion by being surrounded by a conductor in contact with the 

 earth, but it is shielded from the effects of magnetic induction 

 by its sheath of iron. Hence both effects of induction are 

 entirely removed. 



3. They can be neutralized by means of a return wire, using 

 this return wire instead of the earth. If 1 and 2, fig. 4, be 



Fig. 4. 

 >l 



two wires runuing side by side, then the current set up by 

 induction from neighbouring wires in one wire is neutralized 

 by the currents set up in the other side. 



But this assumes either that the disturbing wires are at an 

 infinite distance from 1 and 2, or that 1 and 2 are infinitely 

 near each other. All attempts to use return wires on existing 

 poles, in cables, or in underground wires have utterly failed to 

 do away with inductive disturbance ; but Mr. Bell has had a 

 single gutta-percha wire carrying two conductors made which 

 very nearly fulfils the conditions and gives excellent results. 



The extreme delicacy of the instrument has introduced a 

 disturbance from another cause, viz. leakage. Wires on poles 

 are supported by glass, porcelain, and earthenware insulators ; 

 but the best support that was ever devised is but a poor in- 

 sulator in wet weather. Currents escape over their surface 

 from the wire they support ; and these leakage currents find 



