82 



The outer or secondary coil, consisting of one thousand turns of 

 No. 30 wire, was wound on a wooden spool of such dimensions that 

 the coil itself was 1.7 cm. long and .0 cm. in diameter (inside). The 

 terminals of this coil were connected to a Rowland D'Arsonval galvanom- 

 eter through a key arranged to short-circuit the galvanometer after 

 each throw of the needle. This brought the needle to rest very quickly, 

 and permitted the position of the magnet to be changed without affect- 

 ing the galvanometer. 



The induction coil (one inch) of the transmitter Avas operated by 

 a storage cell and was adjusted to give a 2 mm. spark between two' 

 small brass spheres, one connected to a vertical wire 200 cm. long, 

 the other to earth. The distance between the transmitter and receiver 

 was varied from two meters to twenty meters. The results given iu 

 this paper were obtained when the distance was made five meters. 

 No effort was made to "tune" the circuits. 



The magnet was made from a bar of steel 1.6 cm. square and 3.7 

 cm. long, bent so as to malie a horseshoe magnet about 16 cm. long with, 

 parallel legs 4.8 cm. apart. The primary and secondary coils were 

 fastened in place on a board grooved and graduated so that the magnet 

 could be slid back and forth in the same horizontal plane with, and 

 in a direction at right angles to, the iron core, and placed at any desired 

 distance from it. The graduations extended from to 12 cm., zero 

 distance corresponding to contact between the ends of the magnet and 

 the core. 



To get a reading the galvanometer was first short-circuited and 

 the magnet placed in position. The sliort circuit was then broken, 

 the transmitter operated as long as the deflection of the needle w^as 

 increasing, and the throw observed. 



Table I gives the throws of the galvanometer for the given distances 

 between the magnet and core. 



A. When the magnet is placed 10 cm. from the core and moved 

 one space nearer eacli successive reading. 



B. When the magnet is placed in contact with the core and is 

 moved one space farther fiom it each reading. 



C. When the magnet is removed some distance after each reading 

 and the transmitter operated before the magnet is placed in position 

 for another reading. 



D. When the magnet is turned over (tlie field reversed) between 

 readings. 



