790 Mr. A. Ferguson on the Construction and 



» 



whether such a frame will show any marked tendency to 

 warp — such a point can only be satisfactorily tested by time. 

 In any case the warping can be minimised by following the 

 plan suggested by Gray for a cylindrical bobbin *. 



The instrument under discussion consists of a horizontal 

 baseboard provided with levelling screws, and carrying a 

 horizontal scale on its upper surface ; in this surface are cut 

 two grooves, along which slides a second horizontal board 

 carrying a vertical rectangle about 60x40 cm. in external 

 dimensions. Around this rectangle, which has been coated 

 with insulating varnish, is uniformly laid a thin brass strip 

 1 cm. wide, the ends of the strip being bent outwards at their 

 point of contact, and insulated from each other by a thin strip 

 of mica ; the current is led into and out of the frame by 

 attaching these ends to a spring clip. The magnetometer, 

 which is of the Kelvin type and is provided with a plane 

 mirror, is supported at the centre of the rectangle on an in- 

 dependent table, and by moving the rectangle in its own 

 plane along a line parallel to the base-board scale the 

 variations in the value of ~F/i, and the concomitant variations 



in £— , can readily be compared. 



As described, the instrument may be put to more uses than 

 would be the case were it primarily designed for use as a 

 standard instrument ; and, in such cases, what is gained in 

 comprehensiveness is apt to be lost in precision. Still, the 

 results obtained by the galvanometer in this form will serve 

 to illustrate the points discussed below. 



§ 2. If the needle be suspended at the centre o£ a rectangle 

 of sides 2c and 2d, the perpendiculars from the centre are 

 all equal, and G, the magnetic force at the centre due to 

 unit current in the coil, is given by 



' V cd 



For a square of side 2c, 



G = 4^/2 



§ 3. If a given length of wire has to be wound into a 

 rectangular coil, the square is the least advantageous form, 

 for, remembering that 



c + d = constant (&), 



* Gray, /. c. 



