434 PEOFESSOR W. WATSON ON A DETERMINATION OF THE VALUE OF 
The Helmholtz Galvanometer .—Since the radii of the coils and the distance 
between their mean planes has to be measured, it is important that the radius should 
be as large as possible. It w^as decided that the largest manageable coil wms one 
having a diameter of 60 centims. The construction of a pair of coils of this diameter 
having the grooves in which the wire is to be wound true to within about '02 centim. 
is a problem of considerable difficulty, especially wdren, as in the present case, the coils 
liave to be capable of rotation about a vertical axis, and this rotation has to be 
measured with accuracy. 
The coils were made in the Physical Laboratory of the Royal College of Science 
under my immediate supervision, the construction being enthely performed by 
Mr. J. W. CoLEBROOK, the instrument maker attached to the laboratory, and very 
great credit is due to him for the way in which he acquitted himself of the task. In 
this connection it must be remembered that the tools available w'ere not such as 
would be considered indispensable for such a job in any engineering or instrument¬ 
making workshop. 
Fig. 1. 
In order to measure the angle through which the coils are rotated, it was decided 
to use the azimuth circle of a Kew-pattern magnetometer, and so the coils were made 
to fit the magnetometer and allow of the use of the telescope and scale to determine 
the position of the magnet. The magnetometer employed is one by Elliott, and 
numbered 70. It was used in the magnetic survey of Great Britain and in the series 
