182 



BULLAKD AND MASON 



[chap. 10 



A method depending on the optical excitation of rubidium vapour has been 

 described by SkiHman and Bender (1958). A somewhat similar, and probably 

 superior, device depending on the optical excitation of helium has been 

 described by Keyser ef al. (1961). Both instruments are more sensitive than the 

 proton magnetometer; they are, however, less convenient, and neither has 

 been used at sea. 



Hill has used a proton magnetometer in a fish towed behind a ship. In his 

 instrument the protons in the hydrogen atoms of water are used. The instru- 

 ment now in use is shown in Figs. 3 and 4 ; in it the water is in a bottle containing 



Fig. 3. Proton magnetometer for towing behind a ship. 



550 cm3 and is surrounded by a coil having 4220 turns of copper wire 0.6 mm in 

 diameter. The fish is a cylindrical container with fins to render it stable. It is 

 made of fibre-glass impregnated with an epoxy resin and is 25 cm long and 

 10 cm in external diameter. A current of 1 amp through the coil gives a 

 polarizing field of 320 gauss. 



The fibre-glass fish contains only the bottle and the coil. It is joined by a 

 3-m length of screened cable to a second fish which contains the pre-amplifier 

 and the relay to break the polarizing current and connect the coil to the pre- 



