636 Prof. li. Whiddineton on 



The values of the coils in the grid and anode circuits of 

 this part of the apparatus were so chosen as to produce oscil- 

 lations of about a million frequency (N). 



In order to make obvious any change in N a second valve 

 circuit B was setup close to the one just described and shown 

 on the left of fig. 1. The frequency of this circuit could be 

 adjusted by means of the coudenser 0, so as to be nearly, but 

 not quite, equal to N, so producing a loud audible note in T, 

 the frequency of which could be adjusted to any desired 

 value by a suitable choice of C. 



In order to provide an unvarying standard of pitch to which 

 the note could be adjusted, another valve circuit was set up 

 (inducing into the amplifier), with capacities and inductances 

 so large as to produce an audible note in T*. This note was 

 usually kept constant, and the heterodyne note produced from 

 the N frequency oscillation tuned to it by the condenser 0. 



The musical note circuit is not indicated in fig. 1, but is 

 shown on the extreme right of the photograph of the ex- 

 perimental arrangement (PI. XV.). The two large coils and 

 standard condenser box (maximum value about 1 m.f.d.) are 

 here clearly shown, the valve being hidden behind them. On 

 the extreme left of the bench is shown the metal lined box con- 

 taining a geometrical slide carrying two stiff vertical insulated 

 rods and the attached condenser-plates P, one on each. 



These plates were of polished steel about 5 mm. thick and 

 about 17 sq. cm. area each. 



They were set as nearly parallel as possible by an optical 

 diffraction method, although great care in this respect is not 

 essential to the ultimate working of the apparatus. 



Just to the right of this condenser-plate box is a second 

 box, also metal lined, containing two stiff paper bobbins (each 

 carrying two small inductance coils) and mounted side by 

 side, the left-hand one associated with the parallel-plate 

 condenser P, and the right-hand one with the variable con- 

 denser Of, here shown with a pointer moA r ing over a graduated 

 scale. On the top of this box are seen the two thermionic 

 valves which perform the duty of maintaining the oscillations 

 in these high-frequency circuits. 



In the left top of the photograph is the trumpet of the 

 loud-speaking telephone, while in the centre top are shown 

 the 100-volt lead accumulator units supplying potential to 

 the various valve circuits. 



* The frequency of this audible note could be kept constant to about 

 1 part in 100,000 over a period of several hours. 



t There was also another auxiliary condenser, not shown, of which C 

 was really a fine adjustment. 



