4 Introduction 



Thermometer. No trace, however, of a set of glass plates could be found. 

 It is possible, however, that if the plates were neatly packed up, their 

 small bulk and their apparent uselessness may have enabled them to 

 survive the periodical overhaulings of some less celebrated repository, and 

 that they may yet gain an honourable place in the museum of historical 

 instruments. 



But we need not expect ever to discover a piece of apparatus of still 

 greater historical interest that by which Cavendish proved that the law 

 of electric repulsion could not differ from that of the inverse square by 

 more than ^. It consisted of a pair of somewhat rickety wooden frames, 

 to which two hemispheres of pasteboard were fastened by means of sticks 

 of glass. By pulling a string these frames were made to open like a book, 

 showing within the hemispheres the memorable globe of 12-1 inches 

 diameter, supported on a glass stick as an axis. By pulling the string 

 still more, the hemispheres were drawn quite away from the globe, and 

 a pith ball electrometer was drawn up to the globe to test its "degree of 

 electrification." A machine so bulky, so brittle, and so inelegant was not 

 likely to last long, even in a lumber room. A facsimile of Cavendish's 

 sketch of it is given at page 118. His own account of the experiment, in 

 Arts. 217-234, is one of the most perfect examples of scientific exposition. 



We might also notice the different electrometers, most of them con- 

 sisting of a pair of cork or pith balls, mounted on straws or on linen threads, 

 and some of them capable of having their weight altered by means of 

 wires run into the straws; but though Cavendish had a wonderful power 

 of making correct observations and getting accurate results with these 

 somewhat clumsy instruments, we must confess that in these, the most 

 vital organs of electric research, Cavendish showed less inventive genius 

 than some of his contemporaries. When Lane and Henly brought out 

 their respective electrometers, Cavendish compared their indications, and 

 by stating in every case the distance at which Lane's electrometer dis- 

 charged, he has enabled us to calculate in modern units every degree of 

 electrification that he made use of. What was really needed for Cavendish's 

 experiments was a sensitive electrometer. Cavendish did the best with the 

 electrometers he found in existence, but he did not invent a better one. 



It was not till 1785 that Coulomb began to publish the wonderful series 

 of experiments, in which he got such good results with the torsion electro- 

 meter, an instrument constructed on the same principle as that with which 

 Cavendish afterwards measured the attraction of gravitation ; and it was 

 not till 1787 that Bennett described in the Philosophical Transactions the 

 gold leaf electrometer, by means of which Volta afterwards demonstrated 

 the different electrification of the different metals. 



The electrical machine, by Nairne, was one with a glass globe. 



We should also notice the dividing engine, by Bird, for determining 

 the thickness of the glass plates, and other small distances. 





