750 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
disc was put on a copper cradle covered with paper, supported between the flat poles 
of the Ruhmkorff electro-magnet, with the line joining its principal electrodes at an 
Fig. 57. 
angle of 45° to the magnetic axes of the field, and a current from a large single element 
of Daniell’s was sent through it by these electrodes. One electrode of the galvano- 
meter was applied to the middle of the multiplying branch, and the other was moved 
about on the opposite parts of the circumference of the disc till a position giving no 
current was found, where it was then soldered. The moveable electrode applied to 
different points of the multiplying branch was then found to give sensible galvano- 
meter indications with a motion of a quarter of an inch, and after a very short time 
the zero-point became tolerably steady. The electro-magnet was then made, with 
Fig. 58. 
the galvanometer circuit broken, and when it was closed again a decided indication 
of a current was observed in the galvanometer. This current was checked by sliding 
the moveable electrode towards the end of the multiplying branch next the equatoreal 
