Sine Galvanometer 375 



account of a new instrument, essentially a sine galvanometer for use at large angles, but 

 apparently with the magnetometer fixed to the tripod. This instrument has since been 

 completed by Schuster and F. E. Smith at the National Physical Laboratory of England, 

 but the description of it has not yet been published. A paper on this subject has recently 

 been published by W. Uljanin 1 , but the absolute instrument he has designed has not yet 

 been constructed. A sine galvanometer designed especially for field determinations of 

 the horizontal intensity was described and used in 1920 by N. Watanabe. 2 



6. The essential parts of the design of the instrument now to be described, which is 

 intended for use in the Standardizing Magnetic Observatory of this Department, were 

 completed in 1918, but its construction was not completed until the present year. 

 Photographs of the instrument are given in Figures A and B of Plate 9. It is marked 

 D.T.M. C.I.W. Sine Galvanometer No. 1. 



The base of the instrument, including the tripod, circles, reading microscopes, etc. 

 was originally part of one of Wild's large unifilar theodolites of the latest pattern de-' 

 signed for the determination of the horizontal intensity, and was made by Edelmann; 

 but it was modified in the Department's instrument-shop in three ways. In the first 

 place, a number of the parts, which were found to be magnetic, were replaced by dupli- 

 cates of carefully tested non-magnetic material. In the second place, the mirrors de- 

 signed to illuminate the precision circle were removed and replaced by small, 4-volt, 

 1.2-watt electric lamps in ventilated screening tubes, and the plain glass in the windows 

 beneath them was replaced by milk glass. This has made it possible to set the micro- 

 scopes with much greater facility and with far less error than formerly, as the illumi- 

 nation is now always excellent and is independent of the azimuth. In the third place, 

 two half-m':nute levels were mounted at right angles on the rotating table. 



The instrument has two horizontal circles mounted rigidly together with correspond- 

 ing divisions in the same azimuth and movable together about the same vertical axis. 

 The outer, or rough, circle is uncovered and is divided into intervals of 1. The inner or 

 precision circle is covered by the table which carries the microscopes, the magnetometer, 

 etc. Independent clamps are, of course, provided for the circle and the table, and a 

 tangent screw for the latter. 



The precision circle is 30 cm. in diameter and is divided into intervals of 10'. Two 

 filar micrometer microscopes, set 180 apart, make it readable to less than 2", correspond- 

 ing to one division on the micrometer heads. In addition to the micrometer microscopes, 

 and on account of their small fields, a third microscope, or finder, covering more than 1, 

 is provided, so that the circle can always be read with convenience and certainty in any 

 position. The accuracy of the divisions of the precision circle was tested by measuring 

 an angle of about 5 in each interval of 10, and no error in the total angle as great as 4" 

 was found. The maximum error of division was thus found to be about 2"; the mean 

 error was about 0.6". The eccentric angle was found to be about 30". 



The insulating material used in mounting the lamp sockets and terminals is pyralin. 

 The globes themselves were specially made of non-magnetic material by the National 

 Lamp Works of the General Electric Co. The lamps are shown in Figures A and B of 

 Plate 9 at the bases of the microscopes. 



7. The magnetometer is exceedingly simple in design. The box has the shape shown 

 in Figures A, B, and C of Plate 9, and was cut from a solid block of copper cast in the 

 foundry of this Department by Mr. C. Huff. It is so free from iron as to be diamagnetic. 

 The box is symmetrical about the vertical axis and has two plane faces 4.5 cm. apart. A 

 circular cylindrical hole 2.3 cm. in diameter pierces the box centrally at right angles to 

 these faces and forms the chamber in which the magnet hangs. Glass windows carried by 



1 Terr. Mag. vol. 24, 1919, p. 118. a Proc. Phys. Math. Soc. Japan (3), vol. 2, 1920, p. 210. 



