476 Dr. 0. Lodge on an Electrostatic Field 



tramping about in the daytime appear to be the cause of most 

 of such irregularities as are felt ; but this cause is happily 

 absent at certain hours of the day (or night), and there remains 

 literally nothing but the passage of the North-Western trains, 

 and these are distinctly felt, though only with the most delicate 

 means. The most serious shaking is during a gale. The 

 building then certainly quivers. 



To illustrate the unusual steadiness, it may be interesting 

 to mention that Mr. Chattock rigged up the Cavendish Gravi- 

 tation experiment in my lecture-theatre, taking such precaution 

 as I naturally suggested about suspending from a ball of lead 

 hung from the wall by elastic ; and twice I have successfully 

 exhibited the experiment to a fairly large audience, the first 

 time very well indeed. Convection-currents are the real 

 trouble in this experiment, and these can, by elaborate tinfoil 

 screens, be nearly avoided*. 



After a few preliminary experiments with the new ring and 

 needle, I began seriously to mistrust the effect of the copper 

 near the ring. Sheet copper was used to screen off the wire 

 from the needle, and it was also used all over the outside of 

 the arrangement to screen off stray electrostatic actions around. 

 But I fancied I understood now what the effect of such con- 

 ductors was. The motion of the electricity in them was just 

 of the same sign as the motion of the electrostatic charge for 

 which we were looking ; and if the motion by conduction were 

 permitted, the reaction of the charge thus momentarily redis- 

 tributed on the conducting metal might just mask the direct 

 effect of the same electromotive force on the charged body ; 

 and would mask it, in so far as it was suitably placed for 

 masking it. 



Metals do not screen off the magnetic effect of a moving 

 electrostatic charge (vide Rowland), but they might screen off 

 the converse electrostatic effect of a varying magnetic in- 

 duction. 



On the other hand, by arranging a copper conductor so that 

 the effect of electric charges induced in it by the E.M.F. should 

 assist the direct effect of the E.M.F. on the charged body, then 

 no doubt a deflexion might be rendered visible. 



So I led a short wire round the outside of the ring, and 

 brought its ends into the box where the needle was, one on 

 each side of the needle. 



* Since seeing Mr. Boys's beautiful pocket-arrangement for exhibiting 

 the same effect at the Royal Society soiree this week, I am out of conceit 

 with this apparatus. Evidently the experiment is going to become a 

 commonplace of the class-room. 



