Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/148

of the filaments being attached to the arms of a delicate balance. The axis of the needle thus rolls on two loops of silk fibre, and Dr. Joule finds that its freedom of motion is much greater than when it rolls on agate planes.

Fig. 18. In Fig. 18, $NS$ is the needle, $CC^{\prime }$ is its axis, consisting of a straight cylindrical wire, and $PCQ$, $P^{\prime }C^{\prime }Q^{\prime }$ are the filaments on which the axis rolls. $POQ$ is the balance, consisting of a double bent lever supported by a wire, $OO$, stretched horizontally between the prongs of a forked piece, and having a counterpoise $R$ which can be screwed up or down, so that the balance is in neutral equilibrium about $OO$.

In order that the needle may be in neutral equilibrium as the needle rolls on the filaments the centre of gravity must neither rise nor fall. Hence the distance $OC$ must remain constant as the needle rolls. This condition will be fulfilled if the arms of the balance $OP$ and $OQ$ are equal, and if the filaments are at right angles to the arms.

Dr. Joule finds that the needle should not be more than five inches long. When it is eight inches long, the bending of the needle tends to diminish the apparent dip by a fraction of a minute. The axis of the needle was originally of steel wire, straightened by being brought to a red heat while stretched by a weight, but Dr. Joule found that with the new suspension it is not necessary to use steel wire, for platinum and even standard gold are hard enough.

The balance is attached to a wire $OO$ about a foot long stretched horizontally between the prongs of a fork. This fork is turned round in azimuth by means of a circle at the top of a tripod which supports the whole. Six complete observations of the dip can be