Slow-Speed paper recorder. A recti- 
fier-integrator whose time constant 
can be varied from zero to 23 seconds 
in five steps has been used. 
Figure 6 shows the block diagram 
of an AC amplifier installation and 
Figure 7 shows a typical system fre- 
quency response curve. 
CALIBRATION 
Two methods of system calibration 
are in current use. In one the cali- 
bration signal is induced into the 
detector circuit from an auxiliary 
winding of a few turns. In the other a 
small measured voltage is injected into 
the detector circuit by means of a 
series precision resistor and dividing 
network. The former is conveniently 
applied to compact metal-cored coils 
and the latter to large air-cored coils 
of known area-turns. Obviously the two 
methods behave differently because a 
constant current in the calibrating 
coil introduces a corresponding constant 
change of flux yielding a frequency- 
dependent detector voltage output, while 
a constant injected sinusoidal voltage 
is independent of frequency. 
When the induction system is used 
the effectiveness of the calibrating 
current must be determined by comparison 
with a known flux density. This is 
accomplished by placing the detector 
under test concentric and coaxial with 
a multi-turn coil 200 feet in diameter 
which, in turn, is fed with a measured 
sinusoidal current over the range of 
frequencies desired. A single, large 
diameter coil was selected because it 
was simple to construct, is capable of 
accommodating detectors having a Con- 
siderable range of size and configuratim, 
and is not critical with respect to 
positioning the coil under test. Both 
axial and radial deviations from the 
(axial) flux density within 20 feet of 
the coil centre arg small (less than 2%) 
and known, Vozoff~ has shown theoret- 
ically that the error due to the under- 
lying earth is less than 1% if normal 
conductivities, frequencies below 100 
cps and a coil radius of less than 100 
meters are assumed. This has been 
experimentally verified by noting that 
the effectiveness of the calibration 
current in the auxiliary winding is 
essentially independent of frequency 
and detector orientation. Thus neither 
the effect of proximity to the ground 
nor possible core saturation materially 
invalidate the calibrations. 
330 
It has been standard practice to 
measure the calibration voltage (or 
current) on the DC scale of a calibrated 
dual-beam oscilloscope. Hence it is 
practical to compare the amplifier out- 
put with the input voltage and phase over 
the desired frequency range. 
In order to avoid the simultaneous 
effect of natural signals it is desir- 
able to attenuate the amplifiers by 30 db 
or more during calibration. Their effect 
is further reduced by choosing, for 
routine checks, a calibration frequency 
where the natural signals are usually 
small. 
The calibration signal is applied 
to all of the components at once thus 
affording a convenient check on detector 
polarities. In spite of care, inad- 
vertent transpositions occasionally 
occur. Detailed calibrations are made 
every few days at a time when the back- 
ground is uninteresting, but single fre- 
quency gain checks are made every few 
hours. 
The following precautions have been 
found effective in reducing interference 
introduced through the calibration cir- 
cuitry: 
(i) When a voltage is injected for 
calibration it may be necessary to 
power the signal generator through an 
isolation transformer or to use a 
battery operated model. It may help 
to isolate the oscilloscope too. 
(ii) An induction calibration circuit 
must use its own shielded leads, 
Separated from the signai cables. 
(iii) An RF filter has been required 
on some Calibration circuits. 
(iv) When not actually in use the 
calibration circuit is completely 
disconnected from the signal gener- 
ator, oscilloscope and other compo- 
nents. 
TIMING 
A rated chronometer with electrical 
contacts is used to actuate timing pens 
on the paper charts and to provide 
minute and/or seconds pips on the mag- 
netic tape. Crystal chronometers have 
been obtained and will be used in the 
future. The absolute accuracy required 
depends upon the Signal frequencies and 
the resolution of the chart or magnetic 
tape. Radio time signals are often 
applied directly to one of the tape 
channels. All records are kept in 
Greenwich or Universal time. 
A primary problem in timing is one 
of determining when an event commences. 
