OF 



THE EOYAL SOCIETY. 



" A Determination of the Value of the Earth's Magnetic Field in 

 International Units and a Comparison of the Results with 

 the Value given hy the Kew Observatory Standard Instru- 

 ments." By W. WATSON, B.Sc,, F.E.S., Assistant Professor 

 of Physics at the Royal College of Science, London. Received 

 June 6 Read June 20, 1901. 



(Abstract.) 



A comparison of the readings given by the magnetometers used for 

 measuring the horizontal component of the earth's magnetic field at 

 the various observatories in Great Britain having shown the existence 

 of very marked discrepancies, although the instruments employed are 

 all of the same type, it seemed of interest to employ some entirely 

 different method for the measurement of H, and to compare the results 

 with those given by the unifilar magnetometers. This paper contains 

 a description of such a measurement. The method employed consists 

 in passing a known current through a coil of known dimensions and 

 comparing, by means of a suspended magnetic needle, the field at the 

 centre of the coil, due to the passage of the current, with the horizontal 

 component of the earth's field. The current employed has been 

 measured in two ways : (l)by the deposition of silver in a voltameter, 

 and (2) by a potentiometer method, in which the difference of potential 

 at the terminals of a known resistance is balanced against the E.M.F. 

 of a standard Clark or cadmium cell. 



As a result, it has been found that taking the electro-chemical equi- 

 valent of silver as O'OOlllS, the value of H as determined by this 

 galvanometer method is '00006 C.G.S. unit lower than the value given 

 by the Kew Observatory standard instrument. If, as seems probable, 

 the true value of the electro-chemical equivalent of silver is 0'0011183 

 then this difference is increased to 0-00014 C.G.S. unit. 



VOL. LXIX. 



