693 
eucline 
Adjustment of the multivibrators is best accomplished by adjusting their 
unsynchronized frequency to roughly 0.95 of the desired value, by means of 
the variable resistances in the grids; the synchronization voltage is then 
adjusted to give the correst frequency, Proper adjustment can most easily 
be observed by placing an oscilloscope across the common grid resistance; 
the input impedance of the oscilloscope has no effeat on the operation be- 
cause of the low impedance of this resistance, The voltage across this re- 
sistance shows the grid swings of both sides with the synchronization pulses 
superimposed on them, 
The high-frequency components in the multiyibrator and output signals 
necessitate careful planning of lead and tube locations to avoid cross pickupe 
Other than this, these instruments have caused little trouble in field use, 
They are particularly stable to line-voltage fluctuations, some maintaining 
their accuracy at input line voltages as low as 70 volts rmse Multivibrators 
of this type have been in service for almost a year and have proved quite 
reliable. 
(b) Tuning forks, -- For time standards below | ke/sec, tuning forks 
are the most convenient. Most forks must be carefully shock mounted and 
oriented horizontally in order not to disturb their calibration. The General 
Radio type 813A and 815C forks have performed quite satisfactorily here when 
operated correctly. It should be noted that the larger forks, especially, 
are vulnerable to shock and should be used with cauticn in the region of an 
explosion, 
(c) Stabilized oscillator. -- For field checks on the trequency stand- 
ards, a number of battery-operated, resistance=stabilized L-C oscillators 
were constructed, These were stable to 0.1 percent and could have been used 
as standards if more accurate standards were not available, The circuit for 
these is shown in Fig. 25. 
(d) Miscellaneous circuits, -- The Eccles-Jordan type counter circuits 
have been used to divide down the frequency from some other standard, such 
as a tuning fork or multivibrator, when the standard frequency was incon- 
venient. The time units so produced are usually not simple numbers, a fact 
which is inconvenient in measuring timing records, 
6. Impedance standards 
It is virtually essential that any laboratory which attempts to make 
quantitative electrical measurements have reference standards of impedance 
