136 R H. Hall— Effect of Magnetic Force on the 



be nearly free from error due to such changes. These results 

 are given below in numbers which are merely relative, the 

 smallest being made 100. 



Strip. R. P. on arbitrary scale. Mean. 



No. 5 100+ ) 



'- \ 



5 100 



6 153-4 I 



6 155-3 j 



7 (see below) 131 [ 



7 154 \ 



8 318-3 ) 



328-1 



100 



154-3 

 142-5 

 323-2 



The intensity of the magnetic field in these experiments was 

 probably between 5500 and 6000 (c. g. s.). A rough test in- 

 dicated that the E. P. of No. 5 was about 9000 Xl0~ 6 . 



The two tests with No. 5, which came close together in time 

 agree best. Next in agreement come the tests with No. 6, 

 next those with No. 8, and finally the two trials of No. 7, 

 which came farthest apart in time, are widely divergent in 

 results. The divergence here is so great that a new compari- 

 son of Nos. 7 and 8 was made the next day, the magnetic 

 field being kept for this purpose about the same as on Dec. 

 31st, although later in the day a stronger field was used. This 

 comparison gave 



R. P. of No. 8 1075 323 



or 



P. P. of No. 7 506' 



ult and that of Dec. 3 

 R. P. of No. 8 323 



The mean of this result and that of Dec. 31st gives 



R. P. of No. 7 " 147 



in a magnetic field of about 6000 (c. g. s.). 



The influence of the shape of the specimen examined is quite 

 evident in all the experiments upon Norway iron thus far 

 described. The most -important part of a cross is, no doubt, 

 the region made up of the bases of the arms and the portion of 

 the main strip between and near the arms. This region may 

 be called the effective portion of the cross. All these experi- 

 ments upon Norway iron show that a cross of this metal in 

 which the effective portion has the ratio thickness^- width com- 

 paratively large, shows a comparatively large R. P. This fact 

 is brought out in a particularly interesting manner by the ex- 

 periments of Dec. 31st and Jan. 1st, just described. From the 

 dimensions already given it, is evident that the ratio thickness^r 

 width, is least in No. 5 and greatest in No. 8. In No. 7 it is 

 greater than in No. 5 and in No. 6 greater than in No. 7. 

 Accordingly we are prepared to. find, and do find, that the 



