316 St. John — Wave lengths of Electricity on Iron Wires. 



storage cells, and ran at a fairly constant speed. The armature 

 of the motar was in three sections, and was free from dead 

 points, giving it the great advantage for the present purpose 

 that it could be set in action simply by closing the circuit, 

 making it possible to control it from the observer's station. 



4a. 



F/f 4Jtf Scale- 



The motor was geared to the two-crank shaft K by means of a 

 wheel and pinion. The speed of the shaft K was about 750 

 revolutions per minute, so that about 25 breaks were made per 

 second. The plunging rods was thinned at O so that they 

 were flexible and gave the required freedom of motion; they 

 ran through the bed plate and the brass bar below which served 

 as guides. The plunging rods carried binding screws by which 

 the flexible coils leading the current from the brass post Q 

 were attached. The lower ends of the plungers were of No. 

 18 platinum wire. The brass bottoms of the glass mercury 

 cups screwed into the brass arm N which was adjustable by 

 means of the collar and binding screw L along the pillar P. 

 At P was attached one pole of the battery actuating the coil 



