Computations of the Power of EleBrical MachinfS. 8 7 



The mahogany column A ends in a fquare ^^, upon which the piece K is fitted and (irmly 

 applied, by means of the fcrew and nut exhibited in the feftion. 



To afcertain the power of this machine, Dr. Van Marum relates an experiment made be- 

 fore the Directors of theTeylerian Eftablifliment, and other philofophical gentlemen, in cir- 

 cumflances not very favourable to the apparatus ; but to which he gives the preference, on 

 account of the refpeclability of the affiftants. A battery of ninety jars, each containing up- 

 wards of a fquare foot of coated glafs, was charged to the higheft degree by 150 turns of the 

 plate, fo that it difcharged itfelf. The great Teylerian machine with two plates of fixty- 

 five inches diameter in its original (late, before Dr. Van Marum's improved rubbers were 

 applied to it,never chargedthe fame battery, in the moft favourable circumftances, in lefs than 

 66 turns. It follows, therefore, that this fmall and fimple machine exhibited -p^^^jths, or about 

 |ths of the power of that great machine in its firftftate; and probably, if the circumftances had 

 been alike favourable in each, it would have amounted to one half. The Dodlor has grounded 

 a calculation upon thefe fafts ; but as he ftates the rubbed furfaces of thefe two machines» 

 probably by fome miftake in calculation, to be 1243 and 9636 fquare inches refpedively, 

 I (hall repeat the calculation in this place. 



The diameter of the plate is 31 inches, and the length of the cufliion 9 inches. Then 

 31 .7854 — 31 — ikl=.7854 = 522 fquare inches rubbed by one cufhion on one lide. And 

 522X4=2088 fquare inches rubbed by the four cufhions. Again in the great machine, 

 the two plates having a diameter of 65 inches, and eight cufliions of 15^ inches long, 

 65l^7854— 65^^^^J?'^.7854 = 24io.4. And 2410 4X8 = 19283 fquare inches rubbed. But 

 the intenfity of the electric power of a machine will be in the compound ratio inverfely of 

 the furfaces and number of turns when the charge is the fame ; Or 150X2088 : 66 X 19283 

 : : I = the intenfity of the larger machine : 4 = the intenfity of the fmaller. 



To have increafed the power of fteady excitation four-fold, is certainly an aftonifliing ac- 

 quifition. This expre.Tion, however, of the intenfities appears to be lefs generally ufeful 

 than that of the ratio of the futface rubbed, to that which is charged. This laft exprelhon 

 becomes very fimple when the latter quantity is reduced to i, or unity. Thus, in the two 



machines here mentioned, the rubbed furfaces in inches for the battery are 



90X144 



and ^—, which are equal to the fimple numbers 90.5 and 24.0, which refpeclively 



denote the number of inches rubbed to charge one inch of coated glafs. 



The great machine charged a fingle foot of glafs, by rubbing 66.6 feet ; and a battery of 

 224 feet at the rate of 94.8 feet rubbed per foot. If the gradual decay of excitation be 

 fuppofcd the fame in the fmall machine here dcfcribed, it mult have commenced with an 

 intenfity of 17.6. In the Philofophical Tranfaclions, already quoted, I have (fated the com- 

 mencing intenfity of a cylinder, excited by the amalgam of zinc, as tried by a jar of 2| feet 

 to be 18.03 and I9-34. But, from my notes, I find that this j.ir was charged with lefs than 

 15.0 when the hand was conftantly prelTcd againft the filk-flap ; and alfo, that this prefTure 

 increafed the intenfity as 49 to 39 in fome few trials, not enough varied and repeated. The 

 labour of turning was very great ; much more than, from various circumftances, I am in- 

 clined to fuppofe Mr. Van Marum'j method requires. From this conlideration, as well as 

 from the numbers, and the probability that, on account of the lefs undulation, the charges 

 by a plate may be higher before they explode tiian thofe by a cylinder, and likewife from 



I tlie 



