418 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
3. With this view a cylinder of wood, P, fig. 2, of two inches in altitude and two 
inches diameter, was suspended by two parallel filaments of unspun silk, a b, a! b', fixed 
in the points b b 1 in a diameter of the upper surface of the cylinder at equal distances 
from its centre c, an index i being attached to it, for the purpose of observing on a 
graduated card i g the duration and extent of the oscillations. The threads a b, a' V 
were suspended in a convenient frame, sdes, Plate XXIX. fig. 13, sustained by a firm 
base A B, elevated on levelling screws ; each thread, after passing through fine holes 
in a moveable bar r r', was joined above to a strong piece of silk line a a 1 , continued 
through holes in the fixed bar d e, being finally attached to regulating pegs at d and e. 
By this arrangement the respective lengths of the threads of oscillation a b, a! b' were 
readily adjusted, so as to cause the cylindrical weight P to hang parallel with the 
plane of the card i g, fig. 2. By changing the situation of the bar r d, fig. 13, the lengths 
of the threads of oscillation could be varied ; and by a succession of fine holes in the 
bar r d corresponding to holes in the cylindrical weight P, it was also easy to vary 
their distance apart. 
4. Having obtained in this way a given length and distance between the threads, 
the centre of the cylinder c, fig. 2, was caused to hang immediately over the centre 
of the graduated circle i g, as shown by contact with the finely-pointed extremity of 
a vertical rod, passed through friction-corks in the central block D, fig. 13. Small 
stays, s' s" s'", &c., of light reed or cork being now inserted between the threads at 
given distances (sect. 8. v.), in order to prevent them from collapsing ; the index i 
was turned to an angle of 60°, and the weight P allowed to oscillate ; the fine central 
point being subsequently depressed from beneath the base B B', fig. 13. 
Experiment A. — By carefully noting the rate of oscillation, the following results 
were immediately arrived at, viz. : 
1°. The time of an oscillation is as the square root of the length of the threads of 
suspension divided by their distance apart, and is altogether independent of the 
weight of the oscillating body. 
2°. The oscillations are isochronous at all angles. 
5. From these results we may, by the general formula n — 
P id a 2 , ^ 
employed by 
Coulombe in his experiments on the torsion of wires, easily deduce the laws of the 
reactive force imparted to the threads. 
In this formula, n is the force in terms of a unit of weight, = 1 grain, which, 
applied perpendicularly at the extremity of a lever of a unit of length, = 1 inch, will 
resist the reactive force imparted to the threads, when the cylinder c, figs. 2 and 13 
has been turned about its axis, through an arc of 60°, whose chord is equal to the unit 
of length = radius. P is the weight of the cylinder c in terms of the unit of weight ; 
a its radius ; g the force of gravity, = 386 inches ; T the time of an oscillation in 
seconds ; x the ratio of the circumference to the diameter of the circle. 
In applying this formula it is easy to perceive that the value of n will vary with 
