474 Dr. 0. Lodge on an Electrostatic Field 



any rate, to try a more sensitive arrangement. Hitherto th( 

 suspending-fibre had been horizontal, and the time of oscil- 

 lation fciirly quick, the mirror, as it were, lying on its back, 

 and the light-beam reflected down to it by a shaving-glass. 



All this was now changed. The iron ring was set up ver- 

 tically, and the needle was delicately suspended, care being 

 taken to keep its moment of inertia very small. The ring- 

 was wound, not all over, but only over a sector of some 45°, 

 and this wire-covered portion was then carefully screened by 

 sheet copper from being able to see the needle and thereby affect 

 it electrostatically. The copper did not form a closed circuit. 



The needle was again a pair of scraps of aluminium-foil 

 attached to the ends of a shellac arm holding also a mirror. 

 The charging was done after a proof-plane fashion, and the 

 insulation was pretty good. Asbestos and sulphuric acid kept 

 the air of the box dry. 



Taking suitable precautions and trying the magnetic ex- 

 periment under the new conditions, with the electrostatic 

 effect of the wire carefully screened off, but no other part of 

 the iron ring screened from the needle, an effect was observed, 

 and it was small. But it was irreversible. 



I traced it, after a little difficulty, to Foucault currents set 

 up in the aluminium scraps. One might certainly have ex- 

 pected such currents there ; but I did not expect them to 

 matter, except by reason of heat, because the metal is in no 

 magnetic field — except, indeed, the earth's. However, it must 

 be that some few stray lines escape from the iron circuit and, 

 crossing through the air, affect the currents excited in the 

 metal by the changing magnetic induction. 



So I had to abandon aluminium, and selected very thin 

 mica, silvered chemically, and held curved to the proper cur- 

 vature by silk threads. But even in the thin silver film 

 Foucault currents could still be detected ; so the film was 

 scored regularly all over with a needle-point. But though 

 this diminished the effect to almost nothing, it was still com- 

 parable to what we wanted to observe. 



So I beat about for other conductors, good enough to charge 

 statically, but bad enough not to have currents induced in 

 them. At length I liit upon the thin gelatine stuff which is 

 wrapped round crackers. It was not exactly this, but it was 

 some variety of gelatine that Davies ultimately used, con- 

 structing it into a pair of little cylinders, say ^ inch long by 

 ^ inch in diameter, very light, and only just conducting enough 

 to receive a charge after they had been in the artificially dried 

 air of the glass box for some time. The shellac arm and every- 

 thing were made afresh, and as shght and fragile as possible. 



