374 Special Reports 



Thus if G, J, and 6 are known, H can be calculated from the equation 



H = U-, (3) 



If the constant G can be calculated from direct linear measurements upon the coil, 

 the instrument is known as an absolute instrument. 



3. If the construction is such that tan ^ can be measured, the coil remaining fixed, 

 the instrument becomes a tangent galvanometer. In all instruments of this kind with 

 which I am acquainted, the magnetometer remains fixed like the coil, and the deflection 

 of the magnet and mirror cannot be read, as in the sine galvanometer, with a precision 

 circle. Moreover, the torsion of the fiber which supports the magnet is different in the 

 initial and final positions, since the magnet moves with respect to the magnetometer 

 box. But if the magnetometer (including the reading devices, such as telescope and 

 scale) is constructed to move over a divided circle, and the scale reading is made the same 

 in the initial and final positions, as in the sine galvanometer, both disadvantages (the 

 first not of great importance) are removed and the instrument becomes capable of 

 precise measurements. 



Another, but less flexible, method of rendering a tangent galvanometer precise 

 consists merely in the substitution of a multiple-faced mirror with fixed angles between 

 the faces for one with a single face, and has recently been proposed by W. A. Jenkins 1 . 

 With this device any mirror sine galvanometer can be transformed into a tangent gal- 

 vanometer, but the torsion of the fiber is not eliminated. 



4. Returning to the sine galvanometer, and differentiating (3) logarithmically, we 

 find that the error AH/H in determining H arising from the errors in determining 

 G, J, and 6 is given by the equation 



AH AG . AJ , . ta ... 



= + ~ -cote A6 (4) 



It has long been possible to determine an angle and its sine, and it has more recently, 

 within the last thirty years, become possible to determine an electric current, with great 

 precision and facility. At the same time, by using the method of winding a coil in a 

 single layer in spirally cut grooves on a cylindrical surface, as first suggested by J. 

 Viriamu Jones 2 , it has become possible to construct a coil whose constant G can be 

 calculated with great precision. 



It has thus become possible, as we shall see, to construct a sine galvanometer by 

 which H can be measured with all the precision which is desirable, in view of its known 

 fluctuations, and with a rapidity far greater than that which is possible with the mag- 

 netometer method. 



5. The first sine galvanometer of precision designed for absolute measurements of 

 magnetic intensity was described by W. Watson in 1902 3 . It was made simply by adding 

 a pair of large and carefully constructed Helmholtz coils to a Kew magnetometer, and 

 rendered excellent service in spite of its somewhat cumbersome character. A new and 

 complete instrument, with a single coil, and of much smaller dimensions than Watson's, 

 was designed for the same purpose in 1912 by N. E. Dorsey, while a Research Associate 

 in the Department of Terrestrial Magnetism; but the instrument was never constructed, 

 and no account of it has been published 4 . In 1914 5 Schuster published a preliminary 



' Phil. Mag. (6), Vol. 41. 1921, p. 454. 



' Roy. Boo. Proc. vol. 63, 189.S, p. 204. 



> Phil. Trans. A., vol. 198. 1902, p. 431. 



1 I have had the advantage of reading Dr. Dorsey 'a MS. For a brief reference, see C. I. W. Yearbook, No. 13, 1914, 



p. 322. 

 ' Terr. Mag. vol. 19. 1914, p. 19. 



