Mag neto-Electric Induction. 



457 



In this Table the dark spaces in 2 and 4 mean absence of 

 metal. The sum of the torsional effects of 2 and 3 or of 4 

 and 5 is equivalent to that of the disk in 1 having a concentric 

 circular cut 2 inches or 1 J inch in diameter respectively — from 

 which it will be seen that a concentric circular cut interferes 

 with the induced currents far more than radial cuts of equal 

 or even greater length. Experiment 6 is a repetition of ex- 

 periment 1. The effect of the radial cuts in 7, 8, 9, and 10, 

 though agreeing in kind with the experiments of Babbage and 

 Herschel, is far less in amount ; and this is noteworthy, as in 

 their experiments the disks of lead were not cut completely to 

 the centre, but a small space was left to unite the different parts 

 together ; while in our experiments the copper disk was cut 

 completely through, the different parts being held in position 

 by the ebonite plate above. 



Taking the torsional effect of an uncut disk as 100, the fol- 

 lowing numbers show the effect of disks cut as in 7, 8, and 10, 

 as obtained by 



7. 8. 10. 



Babbage and Herschel . . . 83*2 72*5 44*8 



Us 92-8 80-7 64*2 



V. Effect of Thickness. — The examination of the effect of, 

 thickness is experimentally involved in that of distance, because, 

 in increasing the thickness of a plate, it is impossible with our 

 data to avoid change of distance of plane of effective mass. 

 We have thought it best to employ for this purpose very thin 

 disks cut out of a uniform sheet, and stuck with thin gum 

 under the ebonite disk, at first one, then two, and so on up to 

 six. Tinfoil was used 0*13 millim. thick, and 3*625 in. in 

 diameter. 



Table V. 



No. of foils. 



A. 



T. 



A'=AxT. 



A A'. 



1. 



4-9 



22-7 



111-2 



111-2 



2 



105 



21-3 



223-b 



112-4 



3 



159 



21-3 



3387 



115-1 



4 



220 



21-3 



468-6 



129-9 



5 



26-6 



223 



593-2 



124-6 



6 



300 



24-6 



738-0 



144-8 



\a 



63 



232 



146-2 





The column AA / shows the increase in effect due to each 

 foil; it also shows that the lower foils, being nearer the magnet, 

 are more effective than the upper ones. A single foil la was 

 afterwards hung at the distance of foil no. 6, and gave the 

 result 146*2, agreeing with 144*8. The effect, then, of disks 



Phil. Mag. S. 5. Vol. 8. No. 51. Dec. 1879. 2 1 



