16 Mr Harris on Magnetic Intensity by the 



therefore, might be employed to detect changes in tension, at any 

 time and place, provided the force which induced motion in the 

 needle, and that force by which it would be eventually reduced 

 to rest, without the ring, were both constant quantities. But such 

 is not the case. We may, however, reverse the experiment with 

 advantage, and cause the ring c, Fig. 8, to oscillate about the bar 

 m, which may be conveniently fixed within the ring, so as just to 

 clear it ; and thus a comparative measure of the force of a bar at 

 its poles may be arrived at, by observing the influence of the mag- 

 net in reducing the ring to rest. This experiment may be ma- 

 naged in the following way. 



28. A ring of copper c, Fig. 8, of about 0,4 of an inch wide, 

 and the ^th or the 0,2 of an inch thick, is suspended by two pa- 

 rallel threads of silk fibre over a graduated circle, as shewn in the 

 figure, The threads are fixed in a diameter of the circle, by means 

 of a cross bar of brass a b, and at about the yth of an inch dis- 

 tance on each side of the centre. The ring is to be accurately 

 balanced, and carefully adjusted to parallelism by means of the 

 sliding wires at w, from which it is suspended, and its index points, 

 1, 2, 3, 4, fixed at each of its quadrants, brought exactly over the 

 zeros on the graduated card below. The ring may be suspended 

 by means of the brass plane at w, in the frame-work above men- 

 tioned, Fig. 3, fixed on the bar /', and hence be substituted 

 occasionally for the magnetic bar. 



29. The ring thus circumstanced, is to be deflected by means 

 of the forked lever above mentioned, Fig. 4, from the direc- 

 tion of the parallel threads of suspension ; and again set free at 

 any given angle by means of the forked portions acting on 

 the transverse bar from which the ring is hung. In this case 

 a long and steady state of vibration will be obtained, in con- 

 sequence of the centre of gravity of the mass constantly 

 rising and falling as the ring swings from one extremity of 

 the arc of vibration to the other. Now, if the number of 

 vibrations made by the ring alone, between given points, on the 



