172 



Dr. J. C. Bose. On the Determination of the 



secures a great saving of space. Two jointed electrodes carry the 

 two beads at their ends ; the distance between the beads and the 

 interposed ball can be thus adjusted. This is a matter of importance, as 

 the receiver does not properly respond when the spark-length is too 

 large. Small sparks are found more effective with the receiver used. 

 After a little experience it is possible to tell whether the discharge is 

 oscillatory or not. The effective sparks have a smooth sound, 

 whereas non- oscillatory discharges give rise to a peculiar cracked 

 sound, and appear jagged in outline. 



The wires of the primary coil are in connexion with a small storage 

 cell through a tapping key. The coil, a small storage cell, and the 

 key are enclosed in a tinned iron box. It must be borne in mind 

 that a magnetic disturbance is produced each time the primary 



FIG. 4. 



The Radiating Box, one-fifth natural size. 



circuit of the induction coil is made or broken ; a sudden variation of 

 the magnetic field disturbs the receiver. The iron box in which the 

 coil is enclosed screens the space outside from magnetic disturbance. 

 On one side of the box there is a narrow slit through which the stud 

 of the press-key projects. In front of the box is the radiator tube, 

 which may be square or cylindrical. The radiating apparatus used 

 in the following experiments has a square tube 1 sq. in. in 

 section. The apparatus thus constructed is very portable. 

 The one which I often use is 7 in. in height, 6 in. in length, 

 and 4 in. in breadth. To obtain a flash of radiation it is merely 

 necessary to press the key and then release it. The break is made 

 very sudden by an elastic spring. 



The Spiral Spring Receiver. The receiving circuit consists of a 

 spiral spring coherer in series with a voltaic cell and a dead-beat 

 galvanometer of D'Arsonval type. An account of this form of re- 

 ceiver has already been given (vide " On the Indices of Refraction of 



