SHORT ELECTRIC WAVE RADIATION 553 



limits such variation is not important to the biological investigator. The 

 maintenance of constant voltages across condenser C3, however, is impor- 

 tant and the ckcuits used in this work were designed with this factor 

 in mind. 



The circuit in Fig. 2 is set up with the intention of simplifying the 

 method of obtaining high frequencies for biological experimentation. 

 As far as the mode of oscillation is concerned, this circuit is similar to 

 that of Fig. 1. Instead of two tubes in push-pull, however, we now have 

 one tube, and, at the other end of the inductance loops, a capacitance 

 composed of two plates spaced apart a distance sufficient that the 

 capacitance of the condenser so formed will roughly approximate the inter- 

 electrode capacitance of the vacuum tube at the other end of the oscilla- 

 tory circuit. When this is done, the voltage distribution is a maximum 

 at the terminals of condenser CI and at the grid and plate of the vacuum 

 tube supplying the power to the oscillatory circuit. Plate voltages and 

 grid-bias voltages may be fed to the circuit midway between the tube 

 and condenser, since the voltage at these points is zero. As will be indi- 

 cated later, it is possible to place the experimental material between the 

 condenser plates of CI, and it is unnecessary to have a separate exposure 

 circuit. 



It is necessary in any regenerative type of ultra-high-frequency 

 oscillator that it be mechanically symmetrical. This is done in order 

 to provide the shortest possible connections and to reduce the resistance 

 losses of the circuit to a minimum. Only the best of dielectric materials 

 should be used in order to keep down losses and increase the power avail- 

 able from a given size of vacuum tube. Too much emphasis cannot 

 be placed on the size and kind of conductors used in the construction of 

 ultra-high-frequency oscillating circuits. Copper tubing is considered 

 best since it pro /ides a maximum conducting section for a given diameter 

 of conductor. Precautions in this respect combined with a limited use 

 of dielectric supports will provide a low-resistance circuit. 



In any vacuum-tube-oscillator circuit the function of the vacuum 

 tube is to supply the energy for oscillation. In addition to this the 

 vacuum tubes of ordinary commercial design also become a part of the 

 oscillating circuit they supply, and, as pointed out, increase the difficulty 

 of obtaining high-frequency oscillations. Certain types of tubes have 

 recently been designed in which the detrimental effects of the inter- 

 electrode capacities of the tubes have been reduced. Such tubes are, 

 however, at the present time unavailable, and therefore the character- 

 istics of the tube have to be considered together with the inductances and 

 capacitance relations of the oscillator circuit. 



Mention has been made of the desirability of maintaining, insofar as 

 it is possible, a constant voltage across the plates of the condenser used 

 to expose biological material. At first thought it would seem possible to 



