.»fi$ On iht tifual Mea/ures o/'E.'ciirleity: 



"vihicli the firfl Account was given in our Journal, vol. i. page 83 : and the lafl method, be- 

 ing fubjefl to modification, not only from the ftruCture of the lefs eflcntial -parts of the 

 machine, but alfo from the dimenfions and figure of the apartment in which the experi- 

 jments are-nitfde, has bcen-accordingly very little ufed, Elc£lvicians have, therefore, with 

 confiderable reatfon, been difpofed to avail thenvfelves of the fecond method, according to 

 the fimple coiTiputation defcribed in our work laft referred to, page 87. But to this me- 

 -thod alfo Mr. Cuthbertfon, the conftruilor of the great 'Teylerian machine at Haerlem, 

 oflc;rs ferious objeflions; in.-confequence of which he took the trouble to i-epcat fome ex- 

 periments at my retjueft. Thefe experiments, :together with his obfetvations and fuch re- 

 marks as have occurred to myfelf, will form the fubje(fl- of the prefent memoir. 



The Honourable 'Henry Cavendiflv, Efq., from a feries of experiments upon the charges 

 of eleflrical jars *, has deduced, that the quantities of eleilticity which coated glafs of dif- 

 >ferent'£hapes and fezes will receive with the fame degree of eletlrization, are diredlly as 

 the ares of thecoating, and inverfely as the thicknefs of the glafs; and that, when the in- 

 fteiluties vary, the quantities of ele<3:ricity in like circumftances are nearly as the length of 

 the fpark. Mr.'Guthbertfoii's great experience has led him to modify thefe general con- 

 clufions. I have found that in great intenfities the length of the fpark is much more than 

 in proportion to the charge f ; andfrom fome fa£ls hereafter to be related, there is reafon 

 .to think that a real charge of low intenfity cannot be meafured either by the length of 

 its very (hort fpark, or e^/en by the number of turns of the machine. Eleftricians, in ge- 

 neral, ufe the beft glafs they can procure in their vicinity; whence their conclufions are, 

 'fur the moft part, applicableto one kind of glafs only. But Mr. Cuthbertfon has obferved, 

 that the difterent kinds of yfh'tte glafs, and (till more the green, will require very different 

 ■ quantities of eleftricity to charge equal furfaces and thicknefTes to the fame height. He 

 ihowed me a jar, of which the coating had been cut away until its capacity, as determined 

 by the number of turns of the machine, became equal to that of another fimilar jar of the 

 fame thicknefs. The coated furface of the former of thefe two jars might be eftimated at 

 •more than one-third part lefs than that of the former. Hence the neceflity of fome other 

 teft; of cleftrical power, different from that which includes the dimenGons of the jar as one 

 of its elements, is evident. Mr. C. offers the explofion of fteel wire for this purpofe; the 

 refult of the fa£ls obferved by him being, that equal quantities of eleftricity, in the form of 

 a charge, will caufe equal lengths of the fame fleel wire to explode, whether the jar made 

 ;ufe of be of greater or lefs capacity, within certain limits very eafily comprehended in a 

 loofe verbal defcription. The primary obje£l of Tiis experiments was directed to the efta- 

 blifhmcnt of this propofition. 



July 6th, 1798, I waited on Mr. Cuthbertfon at feven in the evening, and found two 

 eledtrical machines in a flate of aiStivity. The firft confided of a glafs-plate 24 inches in 

 diameter, rubbed by two pair of cufhions, each 5 inches long, proceeding from the extreme 

 of the circumference towards the centre upon oppofite radii, whence the whole furface 

 rubbed in each turn was 1193,8 fquare inches, or 8,29 fquare feet. The other machine was 

 of the fame dimer,fions, but had two plates and four pair of cufhions. Ajar fitted up with 

 Lane's eleftrometer, the balls of which were invariably fixed, was ufed to afcertain the 

 iteadinefs of the excitation. When this jar was applied to the prime conduftor of the 



* Philofophical Tranfaftions, Ixvi. p. 196. f Phil. Tranf. Ixxvi. 



ilngle 



