226 LECTUEES TO SCIENCE TEACHERS. 



Of repulsion electrometers I have lastly to bring before 

 you those of Sir William Thomson. The very remarkable 

 collection of electrometers exhibited in the Loan Collection 

 by Sir William Thomson shows the attention that he has 

 bestowed on electrometry. Electrometers in every stage of 

 development may be seen in his collection, but many of 

 them you will have to go and look at in their place. It 

 was impossible to bring them all upon the table before you ; 

 and much more was it out of the question for me to think 

 of attempting to explain them in the two lectures devoted 

 to this subject. 



Modern electrometry is largely due to Sir William 

 Thomson. His instruments have for the present superseded 

 all other electrometers for practical purposes, such as 

 the testing of telegraph cables during construction and 

 after submersion ; and I wish to call your attention to his 

 admirable report on " Electrometers and Electrostatic 

 Measurements," prepared for the British Association Com- 

 mittee on Standards of Electrical Resistance (1867), and 

 republished in his collected papers on Electrostatics and 

 Magnetism. In that report you will find full information 

 on many points that I cannot do more than allude to, while 

 exhibiting some of the electrometers of the Loan Collection 

 to you. 



We have three specimens of Thomson's repulsion electro- 

 meters before us. Two of them are almost identical in 

 construction, so I have dissected one that you may see its 

 parts. 



The glass case is a thin flint glass bell, the lower half of 

 which is coated inside and outside with tinfoil, like a Leyden 

 jar, except that at the bottom of the inside a part of the 

 glass is left bare. That part is filled with strong sulphuric 

 acid, and connected by a piece of platinum foil with the 

 tinfoil coating. The sulphuric acid performs two functions. 

 It keeps the inside of the case of the instrument dry, pre- 

 venting the deposit of moisture on the glass insulators, 1 



1 It is curious that in almost all even of the most recent text-books 

 on electricity we still find a distinction made between diy air and damp 

 air as insulators. So far as we know at present, no difference what- 

 ever exists. Sir William Thomson has not been able to detect the 

 slightest difference between dry air and damp air as to power of insu- 

 lation by the most delicate experiments. The cause of the widespread 

 fallacy on this subject is, of course, that such conductors as are used 



