268 Dr. Tyudall on the Laws of Magnetism. 



needle coinciding with tlie centre of the ring. The instrnmeut 

 was so placed that the plane of the ring was parallel to the mag- 

 netic meridian ; or^ in other words, so that needle and ring were 

 in the same plane. "\Mien a current was sent round the latter, 

 the needle was deflected to the right or to the left, according to 

 the direction of the current, until a position of equilihrium be- 

 tween the action of the latter and the earth's magnetism was 

 attained. The angle which the needle then made with the mag- 

 netic meridian could be read off on the graduated circumference 

 of the box, and it is easy to demonstrate that the tangent of this 

 angle is directly proportional to the intensity of the current. To 

 protect the needle from local disturbance by the stream, it was 

 so arranged that the copper bands to and from the instrument 

 ran close side by side, and thus annulled each other's action. 

 The current thus proceeded from the battery to one leg of the 

 tangent galvanometer, up this leg, round the ring, acting on the 

 needle in its passage, down the other leg, thence to the magnet, 

 from which it returned to the battery. 



In the course of the investigation, I found it convenient to 

 substitute for the tangent galvanometer above described another, 

 for which I am indebted to the kindness of Professor Bunsen. 

 The ring of the latter is of 16 inches inner diameter, the dia- 

 meter of the needle-box is 7 inches, and the limb is graduated to 

 intervals of 10'. The needle is short, and carries at one end a 

 straight fibre of glass, as fine as a hair, which points to the gra- 

 duated circle. To avoid parallax, the fibre was reflected from a 

 metallic surface underneath ; in reading off, the fibre covered its 

 image. The needle comes quickly to rest ; and by means of a 

 small eye-glass, the angle can be read to 5' with the greatest ease. 



4. We have here the means of exciting the magnet and of 

 measuring the power of the exciting current, but not yet the 

 means of varying the latter. This was effected by a rheostat, 

 which was placed upon the same bench with the magnet. The 

 instrument consisted of a stone cylinder capable of l)eing turned 

 by a handle, liound the cylinder a thin wire was coiled, which 

 offered a powerful resistance to the passage of the cm-rent. By 

 turning the handle, any required length of this wire could be 

 thi'own into the circuit, and the power of the current thus varied 

 at pleasure. 



5. The following is the method which was first pursued in the 

 experiments, and is in substance the same as that usually adopted 

 in cases of this nature : — The iron ball being balanced in the 

 manner before described, a current was sent round the magnet ; 

 the ball M'as attracted, and came to rest on the centre of the sur- 

 face presented to it. Weights were then laid upon the opposite 

 scale-pan until the ball was toi'u away, the weight necessary to 



