VACUUM TUBE CIRCUITS AS A SOURCE OF POWER 
FOR CONDUCTIVITY MEASUREMENTS 
H. A. GEAQUE AND F. PAUL RALSTON 
At the present the possible sources of current for conductivity 
measurements are limited to the induction coil, the high frequency 
generator and the Vreeland oscillator. In the first two the oscil- 
lating current is produced mechanically and the noise from the 
mechanism is apt to interfere with the accurate balancing of the 
bridge when measuring the resistance of the conductivity cell. 
This may not be the greatest objection in the case of the induction 
coil as it has been shown that the current has an undirectional 
value. The Vreeland oscillator is the only apparatus in use which 
produces a tunable oscillating current by means of a rectifier and 
controlled by the capacity of the condenser and the inductance of 
the coils in the circuit. This circuit gives an oscillating current 
whose intregal value is zero, and there are no overtones or mechan- 
ical noise to interfere with accurate work. 
The advantages of such a system led to the development of a 
similar circuit using the vacuum tube as an oscillator. Several 
circuits were found which would give an oscillating current of a 
degree most favorable for conductivity measurements, but it was 
difficult to control the frequency and to get the current in the 
secondary circuit of sufficient strength to be of practical value. 
The circuit shown in figure 36 was found to be the most satis- 
factory. The bulb consists of the grid (G), the plate (P) and 
the filament (F). The filament was lighted by a six volt storage 
battery (A) and 0.6 ampere was necessary. The “B” battery (B) 
has a voltage of 25 to 70 volts, the latter giving a stronger cur- 
rent, and is made by connecting thirty flash light batteries in 
series. The inductances Li and L 2 have values of 100 and 1.3 
millihenrys respectively. The variable condenser (C) has a ca- 
pacity of .0005 micro-farad as a maximum and is shunted by a 
high (“grid leak”) resistance of several megohms. The primary 
(P) and secondary (S) coils were put within the inductance 
coil Lj which substantially increased the secondary current. The 
positive pole of the “B” battery was connected to the plate of the 
vacuum tube and in parallel with this the inductance L 2 , the 
