Instrumental Equipment of Parties 5 



of the suspended magnet, although such an attachment is not deemed absolutely 

 necessary by experienced observers who make use of the fingers instead. The 

 distance from the top of the clamp of the stirrup to the center of the magnet when 

 suspended is 35.5 mm., this being great enough to provide against any appreciable 

 change in level of the suspended magnet for a large change in the value of the 

 vertical component. 



The scale by means of which the position of the collimation line of the magnet 

 is referred to the horizontal circle setting, consists of 60 divisions engraved on the 

 piano-parallel glass diaphragm of the magnetometer telescope. Particular atten- 

 tion has been paid to the engraving of these scales to insure accurate graduations 

 and such suitably selected widths of the engraved lines as to permit of sharp 

 definite readings and estimations a point at times overlooked in instruments of 

 otherwise excellent construction. 



The deflection bar is of rectangular cross-section, 5 mm. thick and 15 mm. deep; 

 it is mounted just below the magnet house and is centered and securely held in 

 place by two slightly tapering pins, one at each side of the house. For centering 

 and protecting the deflecting magnet against sudden changes of temperature, it is 

 mounted in a specially constructed wooden box. The lower end of the magnet 

 centering pin of the deflection box is finished so as to fit snugly in rectangular 

 grooves in the deflection bar, thus insuring the invariability of the deflection 

 distances. For maintaining the vertical plane of the center two broad arms extend 

 down from the metal base of the box the full depth of the deflection bar, against 

 which a heavy spring on the rear side presses them; a suitable counter- weight is 

 suspended on the opposite side of the deflection bar to maintain the level of instru- 

 ment during deflections. 



The total weight of this type of instrument, with its case, is 11 kilograms; that 

 of the type 1 (a) previously used by the Department is twice as much. When 

 account is taken also of the much heavier shipping cases required for the older 

 form the material reduction in total weight for transportation of the present 

 instrument is made still more evident. In transportation the deflection bar 

 is placed in a brass tube and packed or carried with the tripod; the weight of the 

 tripod and bar is 4.25 kilograms, against 7.5 for the tripod and bar of the older 

 form. This type of instrument is shown in Plate 3, Fig. 2. 



The first Kew design (a) of the second type (2) is so well known to magneticians 

 as to make unnecessary a detailed description.^ The declination and deflecting 

 magnet is a hollow steel cylinder fitted with a collimating lens and scale. The 

 suspension material is silk fiber. The auxiliary or short magnet for use in deflec- 

 tions is also cylindrical, but the sighting arrangements are effected by means of a 

 plane mirror, attached at right angles to the magnet, in which the image of an ivory 

 scale is observed. The deflection bar is of brass, in one piece. In the earlier field 

 work of the Department and pending the construction of its own design of instru- 

 ment, use was made, in the magnetic survey of China, of instruments of this type 



* For a more detailed and illustrated description see the article "Magnetometer" by William Watson in the 

 Encydopcedia Brilannica, eleventh edition, v. XVII (386-388). Also Stewart & Gee, Practical Physics, v. 2. 



