SECT. 1] 



THE MAGNETIC FIELD OVER THE OCEANS 



183 



amplifier. This second fish is of brass and is rather smaller than the first. By 

 separating the relays from the bottle, their magnetic effect is made negligible. 

 The coil is tuned to the proton precession frequency by a capacitor whose value 

 can be altered from the ship by a stepping switch. The stejDS in resonant fre- 

 quency are about 50 cycles. The Q of the coil is about 90. 



Precautions must be taken to ensure that the polarizing current is reduced 

 to a very small value when it is nominally turned off. If the field is to be 

 measured to 1 y, a polarizing field of 100 gauss must be reduced to 10~'^ of its 

 initial value ; a single set of relay contacts cannot be relied on to do this ; a 

 second relay is, therefore, provided in the ship, and as an additional precaution 

 the leads carrying the polarizing current are short-circuited by contacts in the 

 fish when the current is off. If the polarizing current is broken sufficiently 

 rapidly, considerable sparking takes ]3lace at the relay contacts and leads to a 

 rapid deterioration in them. This may be avoided by placing a resistance 



Fig. 4. Block diagram of proton magnetometer. 



across the ship end of the cable leading to the coil or by providing a relay on 

 the shij) which reduces the current relatively slowly to about 10% of its initial 

 value. 



The pre-amplifier in the fish feeds into a cable running to the ship. This cable 

 has six copper cores and one steel core to provide for the stress of towing. If the 

 bottle is more than two ship's lengths astern of the ship the magnetic dis- 

 turbance produced by most ships is less than 3 y and can usually be neglected. 

 The variation of the ship's field with course and distance has been discussed by 

 Bullard and Mason (1961). 



In the ship the signal is amplified by an amplifier with a band-width of 

 about 100 c/s and fed to a binary counter which counts 2048 cycles of the 

 proton precession. In order to allow the switching transients to subside, the 

 counter is allowed to count for about a quarter of a second before the 2048 count 

 starts. The beginning and end of the 2048 count start and stop a second binary 

 coded decimal counter which counts the oscillations of an e.m.f. derived from 

 a 100-kc/s quartz oscillator. If N be the number of cycles of the 100-kc/s 

 oscillations counted in 2048 proton periods, the precession frequency is 2048 x 

 105/iV and the field, F, is given by : 



F = 2048 X 105 X 27r/7iV = (48,102.2 ± 0.4)/iV. 



In a field of 0.5 gauss the time of observation is about one second and the count. 



