400 



Mr. R Shida. 



from tlie needle in such a way that the plane of the needle is 

 unaltered by the magnet being removed or replaced when desired. 



The wire to be experimented upon is represented by AA'. It is 

 hung vertically at a distance of 10 centims. from, and due magnetic 

 east of the magnetometer needle, by means of an arrangement of 

 pulleys, P, P', P", and weights, W, W 7 , each weighing about half a 

 kilogramme and attached to one end of the cords, T, T', respectively as 

 shown. In order that the wire may easily be detached from the 

 cords, the other end of each cord, instead of being fastened direcbly to 

 the end of the wire, is merely hooked, by a small brass hook which it 

 carries, on to a loop of cord fastened to the end of the wire. The 

 mode of fastening the loop of cord to the wire was as follows: — A 

 cord 20 to 30 centims. long was made into a loop in such a way as to 

 bring its ends together, and this latter part of the loop, after having 

 been untwisted, was put over the .end of the wire so as to enclose 

 about 5 centims. of it in the untwisted portion, over which portion a 

 thin string was tightly coiled a great number of times. This mode 

 of fastening the cord to the wire allowed a heavy weight to be put 

 on the wire without twisting or bending the latter in the slightest 

 degree. 



BB / is the magnetising coil hung in such a manner from the string 

 T that both its centre and axis coincide with those of the wire AA'. 

 The coil used in the first part of the experiments was composed of 

 only one layer of silk-covered copper wire wound on a straight brass 

 tube of about 6 millims. in its internal diameter ; the length of the 

 coil was 108 centims., its radius was *34 of a centimetre, and the 

 number of turns of wire on the coil was 1,795, and the resistance of 

 the coil including the electrodes was, at 14° C, 3"94 ohms. By means 

 of this coil were obtained the results given in the columns headed 

 1 to 5 of the Table I. It was soon found that the coil just described 

 was quite unsuitable for producing high magnetising forces, and that 

 a modification was necessary. The coil, when modified, was 110 

 centims. long, and consisted of five layers of silk-covered copper wire 

 laid on one above another ; the radius of the innermost layer was 

 •340 of a centimetre, and that of the outermost was '660 of a 

 centimetre, and, therefore, the mean radius of the coil was \50, and the 

 mean distance between any two adjacent layers '08 of a centimetre ; 

 the resistance of the coil, the electrodes included, was 30"8 ohms, at 

 14° C. The ends of the electrodes of the coil were permanently con- 

 nected to the two terminals of a reversing key, K, the other two 

 terminals of which were in connexion with the two electrodes of a 

 Thomson tray battery so disposed that any desired number of cells, 

 from 1 to 60 inclusive, could be placed in the circuit. A tangent 

 galvanometer, Gr, was inserted in one of the connecting wires as 

 shown in the sketch, so that whenever a current is passed through the 



