﻿432 Mr. C. E. St. John on Wave-lengths 



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 bind-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 actual ing the 

 coil and also one pole of the condenser in the base of the 

 coil, and at Q was attached one pole of the coil and the other 

 pole of the condenser. The cups were filled with mercury to 

 a height of 8 millim. and then filled with alcohol to within a 

 few milli metres of the top. They usually required cleaning 

 only after several hours' use, when the surface of the mercury 

 consisted of very fine globules, and sharp breaks were not 

 made at each stroke of the plunger as was indicated by the 

 occasional failure of the spark. The character of the spark 

 depended much upon the exact height of the mercury-cups. 

 The adjustment was best made while the coil was in action by 

 raising or lowering the cups until the spark had a white body 

 and a peculiar snap. 



The plates of the Hertz vibrator were 40 centim. square 

 and fixed at 61 centim. apart. The spark-gap was supplied 

 with platinum-faced balls (3 centim. in diameter), which 

 worked with less trouble than the usual brass ones. The side 

 K M (fig. 2) of the secondary circuit was parallel to the con- 

 ductor forming the vibrator plates, and fixed at 6 centim. 

 distance with its centre opposite the spark-gap. The long 

 sides of the rectangular secondary lay in a horizontal plane, 

 and ran through the centre of the room at a height of 1*6 

 metre above the floor. They were held by their end supports 

 at 30 centim. apart. The induction-coil was 53 centim. long, 

 19 centim. in diameter, and was excited by the current from 

 five storage-cells. A sparking distance of about 6 millim. 

 was most effective in producing oscillations in the secondary 

 circuit. 



The following method was pursued in taking the observa- 

 tions. The interruptor was set in action, the circuit closed 

 through the induction-coil, and an observation taken of the 

 first swing of the needle. The circuits were broken as soon 

 as the needle reached the end of its first swing, and the extent 

 of this excursion was the reading recorded. In accordance 

 with the experience of Paalzow and Rubens, it was found 

 that a steady deflexion could not be obtained, but this first 



