210 NOTICES CONCERNING PHILOSOPHICAL APPARATUS. 



t 



XIII. 



Notices concerning fome Philofophical Apparatus. — Electrical Bat- 

 tery of Talc. — Complex Ilorfejhoe Magnet. — Mr. Davy's 

 apparatus, fyc. 



Battery of talc. SEVERAL years ago (in 1788), when I was bufied in elec- 

 trical refearches, I was induced to conftruct a battery of talc, 

 in confequence of the very great capacity of that fubftance 

 which I found (conformably to the determination of Mr. Ca- 

 vendifli with glafs), to be inverfely as its thicknefs. As our 

 galvanic refearches now feem to give additional intereft to ap- 

 paratus of great capacity and low intenfity, I have thought it 

 might be acceptable to give a fketch of it. 

 Jfftimate of the The full charge of two fquare inches of talc gives a fhock 

 elcclnca! capa- fufficiently fevere to be felt above the elbows, giving a fpark 

 is as the thick- of above one tenth of an inch long, and requiring the friction 

 nefs inverfely j f more than one foot of glafs to charge it. I compared thif 

 that'ofgTafs? 3 ta * c w ' tn a ' ar S e j ar °f 351 fquare inches, and 0.082 inch 

 thick, and the talc was 0.01 125 inch thick. The communi- 

 cation from plus to minus was made by one tingle wire, and 

 Lane's electrometer was fet fo that the talc exploded regularly 

 at each turn of a feven inch cylinder. The jar was then placed 

 in the circuit, and exploded at twenty-one turns. Their ab- 

 folute capacities were therefore as 1 to 21. But the furfaces 

 are as 1 to 175, whence the capacities of equal furfaces were 

 as 1 to y r s , or about 1 to 8. And the thicknefles are in- 

 verfely 1 1 and 82, whence it appears that the, actual capacities 

 proved rather in favour of the talc, if* taken from the inverted 

 ratio of the thicknefs. This, however, may be only ap- 

 parent, becaufe the talc was more rapidly charged, and had 

 an infulated rim of only 0.4 inch, whereas the glafs had a bare 

 rim of 4.0 inches, in addition to which it may be obferved, 

 that the uncertainty of the refult is not quoted, than the un- 

 avoidable errors from the admeafurements of the thickneffes 

 and the unfteadinefs of excitation. 

 Drawing and Plate XI. Fig. 1 . reprefents a battery compofed of 12 pieces 



defcription or* a ^f talc, each meafuring 2.6 inches in the fide, and coated on 

 inche! fquare^ a ^ ur ^ ace °* tvv0 % iare inches. The thicknefs of each piece 

 and equivalent to was 0.0025 inch, and consequently its capacity may be taken 

 ofglaft uarefeet at about 20 times that of thin window glafs of one-twentieth. 

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