260 



MendenJiall and Waidner 



sensibility would continue to increase as the magnets decreased 

 in length. This not being the case in general, it is possible to 

 use too short magnets. To show this we have calculated the 

 sensibilities to be expected with four systems made of the 

 same material and cross section as magnets I 2 (see table), which 

 showed a greater intensity of magnetization than any others of 

 the same length. Furthermore we assumed that only four 

 such magnets were to be used in each group, to be m 65 mm long 

 in the first system, and l ram , l'5 mm , 2 mm m the others ; that the 

 mirror weighed "23 mg., and was l mm x l'l ram in area, the staff 



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and shellac weighed "09 mg., and the hook '06 mg. — these 

 values being found by trial. Except for the omission of the 

 hook, which is readily dispensed with, it would be difficult to 

 reduce the non-magnetic mass much below this. The results 

 are shown in curve 2 — where ord mates are proportional to 

 sensibility, and abscissae to length of magnets used. A very 

 decided maximum is shown, for a length of l*l mm . It must be 

 said, however, that the magnets could be put closer together 

 without undue demagnetization, and that two sets could be 



