ELECTRICITY. 



53 



(195.) The instruments called Dou- 

 blers are so contrived, that by executing 

 certain movements, very small quantities 

 of electricity communicated to a part 

 of the apparatus may be continually 

 doubled, until it becomes perceptible 

 by an electroscope. The first invention 

 of this kind was that of Mr. Bennet, 

 which consists of three brass plates, 

 which we shall call A, B, and C. The 

 plate A has an insulating handle fixed 

 in its centre, while the piate B has a 

 similar handle fixed in its circumference. 

 The under side of A, and both sides of 

 B are covered with varnish. The third 

 plate C is also of brass, and is only 

 varnished on its upper side, the lower 

 side communicating with the gold-leaf 

 electroscope. The body whose electri- 

 city is to be tried, is made to communi- 

 cate with the under side of the plate 

 C, which touches the electroscope, 

 while B is placed upon C, and then 

 touched with the finger : the communi- 

 cation with the electrified body is then 

 removed, and B is lifted up by its insu- 

 lating handle. A is then placed, by 

 means of its handle, upon B thus ele- 

 vated. A is then touched, and, after 

 withdrawing the finger, is separated 

 from B. In this process B acquires an 

 electricity contrary to that of C ; and A 

 an electricity contrary to that of B, that 

 is, the same as that of C. If the plate 

 A, thus electrified, be next applied to 

 the under surface of C, and B be again 

 applied over C, and touched with the 

 finger as before, it will be acted upon 

 by the electricities contained both in C 

 and A, and thus acquire, by induction, 

 nearly double the quantity which it had 

 done in the first operation. The con- 

 sequence of this will be that nearly all 

 the free electricities of A and C will be 

 concentrated in C. A may now be re- 

 moved, and after withdrawing the finger 

 from B, B may also be removed, and 

 C will be left with double the quantity 

 of electricity which it had received from 

 the body with which it was originally 

 ,made to communicate. 



If after this duplication the electricity 

 of the plate C be still too feeble to be 

 indicated by the electroscope, the same 

 series of operations must be repeated 

 ten or even twenty times ; when by 

 doubling it every time, the smallest 

 conceivable quantity of electricity must 

 at last be rendered sensible ; since, at 

 the end of the twentieth operation, it 

 will be augmented more than 500,000 

 times. Although the frequent repeti- 

 tion, of the operations may p-ppear tedi- 



ous, yet, by a little practice, the art is 

 readily acquired, and the whole process 

 need not occupy a minute. Great care 

 must be taken in conducting these ex- 

 periments, not to excite any electricity 

 by the friction of the finger, or by any 

 other means, in the varnished sides of 

 the plates. In order to obviate this 

 source of error, Cavallo contrived a 

 form of the instrument, that enabled 

 the plates to be brought within a veiy 

 small distance of one another, yet 

 without actual contact, so as to enable 

 him to dispense altogether with the em- 

 ployment of varnish. But notwith- 

 standing every precaution of this kind, 

 it is always found that the instrument ex- 

 hibits electricity of itself, although none 

 has been previously communicated to 

 it : so that its indications cannot be at 

 all depended upon for the detection of 

 very minute quantities of electricity. It 

 is unnecessary, therefore, to describe the 

 particular mechanisms invented by Dr. 

 Darwin, and improved by Nicholson, 

 for bringing the plates into the requisite 

 positions, and effecting in succession 

 the necessary contacts, by the simple 

 rotation of a winch, aided by wheel- 

 work : instruments which have gone by 

 the names of the moveable, or revolving 

 double?', and the multiplier of electri- 

 city, and which are now superseded in 

 practice by instruments more sensible 

 and certain in their operation. 



CHAPTER XIII. 



Developement of Electricity by Changes 

 of Temperature and of Form. 



(196.) THERE are certain mineral bo- 

 dies, which, from being in a neutral 

 state at ordinary temjperatures, acquire 

 electricity simply by being heated or 

 cooled. This property is possessed only 

 by regularly crystallized minerals ; and 

 of these the most remarkable is the 

 tourmalin, which is a stone of consi- 

 derable hardness, found in many parts 

 of the world, and particularly in the 

 island of Ceylon. The Dutch, who 

 first became acquainted with it in that 

 island, gave it the appellation of As- 

 chentrikker, from its property of attract- 

 ing ashes when it is thrown into the 

 fire. It appears from the researches of 

 Dr. Watson, that its attractive proper- 

 ties were known to Theophrastus, who 

 describes it under the name of Lyncu- 

 rium. Linnaeus has termed it the Za- 

 pis Electricus, (Electric stone.) The 

 form of its crystals is generally that of 

 a nine- sided prism, terminated by a 



