On Eapicl Periodic Variations of Terrestrial Magnetism. 5 



neces>^aiy reconstruction of the room. In any future work of the 

 kind, it will he ahsolutely neccessary to provide first of all for the 

 removal of this obnoxious humidity. 



o. Instruments. — Since it was the immediate ^purpose of 

 the present investigation to detect the most minute disturbances 

 possible, the sensilulity of the usual instruments was far from 

 being sufficient. To meet this need. Prof. Tanakadate and Dr. 

 Kadooka devised a specially sensitive set of instruments which 

 will l>e separately described in the following paragraphs: 



a) Wed- East- or Y- Component Instrument (West taken as 

 positive). Maxwell, in liis discussion of the theory of bifilar 

 suspension^^ suggested the utility for the measurement of the WE- 

 component, of a suspended magnetic needle twisted nearly 180 

 from its natural direction, which can be made highly sensitive by 

 properly adjusting the breadth of the bifilar suspension. This 

 principle was adopted in the following manner. The magnetic 

 needle (Fig. 2, N) with a length of 20 mm. and a diameter of 1*5 



Fis. 



VI w 



Hr^-^ft 



i__t 



t 



2 3^ scr: 



mm., was held by a hook AB, made of aluminium wire of O'T 

 mm. diameter, bent in the form as shown in the figure. A kind of 

 light stirrup for supporting the hook with the magnetic needle 

 was made with pieces of thin fused quartz rods, welded together 



1) Maxwell, Electricity and Magnetism, 3rd. Edition, Vol. II, p. 118. 



