8 AKT. 8.-H. NAGAOKA AND K. HONDA: 



cut uloiiii,' the ]j:ir ; tlie iiToovc is .slii;htlv witleiied ;it tlic iniddlt* of 

 the bar, so as to allow tlie ovoid o to be so placed that the line of 

 its axis ])asses throug'h the centre of the optical lever (a). A small 

 rectangular brass pillar p is erected at one corner of the bar. A 

 small vertical \' -groove is cut in it, and on this two points of tlie 

 optical lever rest. The lever is a small rectangular piece of brass Mdth 

 three steel points, of which two rest on the pillar p, and the remain- 

 ing one is in the same line as the axis of the ovoid, and rests on small 

 plane glass plate, which is fixed to the end of the mcjvable brass rod 

 />. The lever arm is e<[ual to 1.168 mm. Preliminary testing shou' 

 ed that the relative positi(.^ns <jf these three steel points were not 

 directly affected by the magnetizing force. The lever is pulled on 

 both sides by small spiral springs of hard brass wire. These springs 

 can be adjusted by slide arrangements, so that the lever turns its two 

 points about on p, without being pushed out by the displacement of 

 the ovoid. The reflecting face of the prism F attached to tlie lever 

 was silvered. The o-reatest difficultv in the measurement of changfe of 

 length by magnetization arises from the temperature chfinges produced 

 by the magnetizing current. The temperature effect can, however, 

 be greatly compensated for l)y applying the principle of the gridiron 

 pendulum. This end was achieved by using zinc rods of suitable 

 length, so that, in any combination, the total expansion due to small 

 changes of temperature in particular lengths of zinc and the ovoid 

 was equal to that in a particular length of brass. Zinc rods 5 mm. 

 thick were carefully turned on a lathe and cut into proper lengths. 



A brass rod (/>) 5 mm. thick is placed in contact with the zinc rod 

 {z). At the end of the lirass rod, a plane glass plate is attached, so 

 that the steel point of the lever comes in contact with it. At the other 

 extremity of the row of rods and the ovoid is a st(j]). It consists of a 

 triangular prism of brass to which a brass rod (d) 5 mm. thick is 



