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X. A Determination of the Yalue of the EartKs Magnetic Field in International 
Units^ and a Comparison of the Results with the Values given hy the Kew 
Observatory Standard Instruments. 
By William Watson, A.R.C.S., B.Sc., F.R.S., Assistant Professor of Physics 
at the Royal College of Science, London. 
Received June 6,—Read June 20, 1901. 
The discrepancies found by Professor PtiCKEii and the author* to exist between 
the values for the horizontal component of the earth’s magnetic field, as measured 
by the absolute instruments in use in the various British observatories, were so 
great that it seemed of interest to measure the field at one ot the observatories 
by an entirely different method, in order, if possible, to obtain some indication as 
to the reliability of the various instruments. 
Further, in the ordinary method of measuring H, a correction has to be applied on 
account of the distribution of the magnetism in the magnets employed, about the 
value of which there is some uncertainty, t 
It was therefore decided to make a measurement of the horizontal component ot 
the earth’s field, by comparing it with the field produced at a certain point by a 
known current flowing through a coil of known dimensions. The comparison was 
to be made by suspending a small magnetic needle at the centre of the coils, 
and noting its deflection when acted upon by the earth’s field and the field due to the 
coils. In this way a direct comparison would be obtained between the value of the 
unit magnetic field, as deduced from absolute magnetic measurements, and the value 
of the unit field produced by a known current, the value of the current being 
deduced from an entirely different set of measurements from those used in 
determining the constants of the magnetometers. The following paper contains 
an account of such a comparison. 
For constructional reasons, as well as to secure a uniform magnetic field at the 
centre of the coil where the magnetic needle is suspended, and so to reduce any 
error which any want of accuracy in the centring, or any departure from the 
(a 309.) 
* ‘Brit. Assoc. Rep.,’ p. 87, 1896. 
t Chree, ‘Roy. Soc. Proc.,’ vol. 65, p. 375, 1899. 
2.5.02. 
