the Electricity given by Induction-Machines. 71 



apparent exception is a strong confirmation of this rule — the case of 

 G. H (I + K). Its observed 3>=3*0552, while the sum = 4*G220. 

 G and I being on the same primary are excited together; but in 

 measuring either, as there is no current in the other (but merely a 

 state of tension), their n is not changed. When, however, they 

 are connected collaterally, the currents react on each other, their n's 

 are increased : the <E>'s are thus diminished, and therefore their re- 

 sultant is the sum of quantities less than those used in the computa- 

 tion. The fl's in this case become—for G 0*94289, for H 079885, 

 and for I + K 0*34588, which are quite sufficient to account for the 

 difference. 



3. As with I in Table II., so here it will be observed that N has 

 less relative power than either I or K ; its actual power is even less, 

 though its theoretical force exceeds theirs in the proportion of 5 : 4. 

 This is explained by its b being so much smaller ; but it gives this 

 important information, that, at least in helices of these dimensions, 

 nothing is gained by using wire thicker than that of I, or -^ inch*. 



4. The effect of L is far less than that of I + K. In the first the 

 helices are on the same primary, in the other on separate ones. In 

 the former case the II is larger, for it is the sum of the II of each on 

 itself, and those of each on the other ; b therefore is less. Besides, 

 the potential of the core on the helices is less than when each of them 

 is central on it. 



The difference is even more remarkable in O as compared to its 

 elements M + N, its effect being only 0*7 of the other, and 0'3 of 

 the theoretic power. The same disparity of course prevails in their 

 combinations; O . L giving only 3*46, while the same four helices 

 arranged on separate primaries give 8*19. The combinations 

 G . H (I + K) and G . L . H have the same helices ; but in the first 

 two were on the same primaries. As, however, they were I "5 inch 

 instead of 0*5 apart, the II was not so much increased as in the other 

 cases, and therefore there is not quite so great a decrease of power. 



The following practical maxims may be deduced from the expe- 

 riments and reasoning which I have related. 



The attention of instrument-makers has been chiefly directed 

 towards increasing the length of spark given by these machines, and 

 in this they have succeeded to a surprising degree ; but in doing 

 so they have not added to the quantity of electricity which is pro- 

 duced by them. This, however, is by far the most important object ; 

 for in most applications of the inductorium all tension above what is 

 necessary to force the necessary quantity of current through the cir- 

 cuit is useless, nay sometimes injuriousf. I am inclined to think 

 that a tension which gives sparks of 4 inches will be found quite 

 sufficient in ordinary cases, and this will be given by about 20,000 

 spires ; all beyond only adding to the weight of the instrument, 

 its cost, and the difficulty of insuring its insulation. It must be 

 kept in mind that the mere quantity is independent of the length 



* This is between 27 and 28 of the Birmingham wire-gauge. I believe Euhm- 

 korff uses 28. 



t It has often been remarked that intense discbarges will not show strata well 

 in an exhausted tube. 



