FORCES ACTING IN MAGNETIC CIRCUITS. 911 
17. We can make a comparison between the work done by a 
ring magnet when it is divided at one point, and the work done 
when the ring is divided at two points, but the reluctance data 
show that though the mean air gap reluctance may be larger than 
that of the iron, itis not very greatly so in any practical case, and 
we can therefore obtain no information by supposing that one is 
much greater or less than the other, but must proceed by actual 
trial from the curves to find out which is the most efficient 
arrangement. 
18. In the case of a mechanism represented by a ring divided 
at one point only, we must remember that this involves a “sliding” 
magnetic contact, and if friction on the bearings is to be avoided, 
this practically ties us down to iron of symmetrical form. 
19. Incidentally, I had occasion to observe the change of reluc- 
tance caused by cutting a bar and then grinding and polishing 
the ends. The reluctance corresponded to a separation of the bars 
by about twenty wave lengths of sodium light, but I am certain 
that the bars could not have been half so far apart as this, so the 
‘surface reluctance is still unaccounted for. 
