1884.] 



Tlieory of the Magnetic Balance of Hughes. 



319 



no friction, and prevents the moment of inertia of the scale -pan and 

 load interfering, by means of a rotatory motion, with the rapidity with 

 which the pointer comes to rest when a load is pnt into the pan. The 

 position of the pointer P, which revolves when a weight is placed in 

 the scale-pan, is read off npon the spiral scale, D, which, in this 

 specimen, we have gradnated in pounds. In another of these weigh- 

 ing machines, shown in fig. 10, the arrangement is the same with the 

 exception that a cylindric scale D is fixed to the end of the spring 

 and turns with it, the pointer P fixed on the frame of the instrument 

 points to an indication of the weight on a spiral line drawn on the 

 cylinder D. This second arrangement allows us to employ springs 

 whose ends have a relative motion of five or six revolutions. 



We have also made certain weighing machines where the weight is 

 placed on a pan resting on the top of a rod passing up through the 

 spring, and attached to the bottom of the spring, but hitherto we have 

 found that this arrangement introduces too much friction. 



When a very long spring is required for any purpose, and where the 

 weight of the spring itself would cause greater stresses on the upper 

 part than on the lower part of the spring, it is our custom to let the 

 breadth and thickness of the strip of material remain unaltered, but to 

 let the diameter of the coils diminish towards the upper parts. The 

 formulas already given suffice to show how this may be done so as to 

 obtain uniformity of maximum stress at all sections. 



We also present before the Society a model showing a combination 

 of bifilar and spiral spring suspension, in which great rotation and 

 small axial lengthening or shortening are produced by an axial force. 



III. " Note on the Theory of the Magnetic Balance of Hughes." 

 By Professor SlLVANUS P. THOMPSON, B.A., D.Sc, Univ. 

 Coll., Bristol. Communicated by Professor D. E. Hughes, 

 F.R.S. Received February 7, 1884. 



[1.] The magnetic balance recently described by Professor D. E. 

 Hughes* promises to be so convenient and useful an instrument in 

 the laboratory, that the theory of its action and graduation deserves 

 attention. In the actual instrument constructed by Hughes the 

 graduation was empirically determined for a number of values, the 

 remainder being found by interpolation. The instrument consists of 

 a small suspended needle lying in the magnetic meridian provided 

 with a zero-mark placed upon a platform in which a horizontal groove 

 is cut magnetically east and west. In this groove the piece of iron 



* " Proc. Eoy. Sob.," vol. 36, p. 167. 

 VOL. XXXVI. Z 



