ELECTROMETERS. 223 



no trace whatever of electrification on the walls or else- 

 where within. The most refined modern experiments 

 lead to the same result ; and it is evidence of this kind that 

 enables us to regard the law given by Coulomb as holding 

 with extreme exactness. 



In connection with experiments of this kind Faraday 

 used the torsion balance and made great improvements 

 in it. He used the torsion balance in those great 

 researches in which he investigated the curved lines of 

 force, and worked out the theory of induction, and proved 

 that electrification by induction is not what it was sup- 

 posed to be action at a distance but that it is electric 

 disturbance transmitted by means of contiguous particles 

 of the di electric, or insulating medium, between two electri- 

 fied bodies One of the modifications that Faraday made 

 in the torsion balance was the protection of the movable 

 parts of the instrument from external influence. He 

 showed that the indications of an instrument such as the 

 torsion balance, or common gold-leaf electroscope, are per- 

 fectly untrustworthy unless the most careful attention is 

 paid to this matter ; and it is curious that though it is now 

 forty-five years since Faraday pointed this out, instrument- 

 makers have failed, except in very few cases, to give any 

 attention to his warning. For instance, here are two 

 well constructed instruments a Peltier's electrometer and 

 a torsion balance, and neither of these has the slightest 

 protection. Out of all the instruments for teaching pur- 

 poses that we see in this magnificent Loan Collection, 

 electroscopes, electrometers, and so forth, you will find 

 not one in a hundred to have any pretence of protection 

 whatsoever. 



I have here a very simple experiment for illustrating the 

 importance of guarding the movable parts of your electro- 

 static instruments from external influence. Here is a 

 glass bell-jar, from the roof of which hangs a fine glass 

 fibre carrying a horizontal arm of aluminium. The inside 

 of the bell-jar is very clean, and is kept dry with the aid 

 of a dish of strong sulphuric acid. It thus gives excellent 

 insulation, and the aluminium needle, which was charged 

 some hours ago, has, I have no doubt, retained its charge well. 

 I now dry the outside of the bell-jar by passing over its 

 surface a wire hoop, covered with cotton wick, and flaming 



