66 Royal Society : — Rev. T. R. Robinson on increasing 



Electromagnetic Field," Phil. Trans. 1865) ; and the term b is always 

 less than unity. "When helices are consecutive their II' s are added, 

 not when collateral. 



From this it follows that 7 may be neglected : that — ^— - is 



fx o fi—b 



nearly unity ; and that the difference between F and $ increases as 

 b diminishes. 



When equal helices are consecutive, b as well as F are unchanged ; 

 therefore so is <£>. 



When they are collateral, each separate b remains unchanged 

 (unless they be so close that they react on each other) ; and there- 

 fore, as with F, the resultant <E> is the sum of its components. 



If the resistance of the wire be diminished by increasing its section 

 without making much change in the dimensions of the helix, b is 

 diminished, and therefore the coefficient of F. It is evident from the 

 form of equation (c) that «3> has a maximum for r, and that beyond 

 this there is actual loss of power in increasing the thickness of the 

 wire. 



It remained to test these views by experiment, but the task has 

 some difficulties. A single discharge of inductive electricity is usually 

 determined by the swing which it causes in a galvanometer needle ; 

 but it is scarcely possible to get two discharges exactly equal. The 

 slightest variation in the manner of breaking the circuit, the least 

 oxidation or roughening of the surfaces where the break is made, 

 change the result ; and therefore it seemed best to take the actual 

 working of the inductorium, in hopes that the average of some thou- 

 sand discharges must be near the real value of the current. 



The rheometer which I used is Weber's (for the use of which I 

 am indebted to the kindness of Mr. Gassiot), and it showed an 

 amount of fluctuation even greater than I expected. With every 

 precaution as to the action of the rheotome, the mirror of the Weber 

 never becomes stationary, and the oscillations are irregular ; twelve 

 of them were taken for each set, of course read at each end and 

 reduced by the usual formula ; yet the sets differ so much, that I 

 only offer their results as tolerable approximations. Two facts illus- 

 trating this uncertainty may be mentioned. With a mechanical 

 rheotome driven at a uniform speed, and its acting surfaces platinum, 

 the ratios of the current were — 



When set so that the point rises 1 1*0000 



but little from the anvil .... J 



Rise greater ] '7894 



Rise still greater, tension of spring greater . . 1*9685 

 Rise still greater, tension further increased . . 1*8371 

 Here a slight change of the adjustment nearly doubles the action 

 of the inductorium. 



Another cause of uncertainty is the variable speed of the rheotome. 

 In general it is worked by the primary current, and therefore is 

 affected by fluctuation of the battery and the extra current of the pri- 

 mary. The mechanical rheotome which I have mentioned is driven 

 by clockwork, and its speed can be exactly regulated. With it I 

 obtained 



