â88 



A. TANAKADATE 



the width of the jaw was ^b and the deflection was limited to }^a, 

 so that the error fell within ^^ for any reading even if w^e supposée 

 the eccentricity in the direction of y to be as much as ^ b. 



To verify the results thus far obtained the following experiment 

 was made in the Physics Laboratory of the Science College of the 

 Imperial University : — Six blocks of wood, each 21 cm. high and 13 

 cm. wide were arranged in a row upon a long laboratory table 

 extending through a space of 3.7 metres along the magnetic me- 

 ridian. These blocks served simply as guides for the stretching of 

 two rectangular coils of insulated wire, whose distance apart bore to 

 the height of either the ratio required (tangent 30°). Each coil con- 

 sisted of six turns. Thus were obtained two parallel magnetic strips 

 of practically infinite length. 



One of Thomson's graded galvanometers with its field magnet 

 taken away, was placed in the space between the two central blocks, 

 its V-groove lying along the magnetic east and west line. The height 

 of the galvanometer was so adjusted that the centre of the four small 

 magnets, belonging to the fan-shaped compass, was always in the 

 plane half-way between the upper and the lower lines. 



A current was run round the wire and was left for about 20 

 minutes till its flow became steady, and then the compass was slid 

 along the V-groove and its position and deflection simultaneously 

 observed at several positions. From these observations the curve I 

 in Fig. 7 was obtained. By a slight modification of the arrange- 

 ment, providing a vertical V-groove, the compass was made to 

 move along a vertical line ; and from a similar series of observa- 

 tions the curve II was obtained. These should be compared with 

 the curves of Fuj. 6. 



