ia) 



(&) 

 Fig. 8-55. Arrangement for magnetic model experiments 

 (after Barret), (a) Energizing coil, model tray and in- 

 ductor platform, (6) inductor. 



tray may be changed, 

 rather accurate depth 

 determinations are 

 possible with this 

 method (see Fig. 

 8-55). 



Later Jenny de- 

 vised an apparatus in 

 which the shape of a 

 structure was repre- 

 sented by bar mag- 

 nets suspended from 

 pulleys with counter- 

 weights. The vector 

 direction was indi- 

 cated by small dipping 

 needles free to move in 

 vertical and horizontal 

 planes. In place of 

 the bar magnets, glass 

 tubes filled with mate- 

 rials of different sus- 

 ceptibility were sug- 

 gested to represent the 

 vertical sequence of 

 formations of differ- 

 ent magnetizations. 

 In that case the nor- 

 mal terrestrial field 

 was produced by a 

 coil. 



G. Underground, 

 Aerial, and Plat- 

 form Surveys 



Since magnetic sur- 

 veys underground are 

 greatly handicapped 

 by power fines, pipes, 

 and rails, such objects 

 should be removed as 

 far as practicable. Er- 

 ratic readings may 

 be produced by ore 

 stringers and strongly 



lo^Terr. Mag., 40(1), 71 (1935). 



404 



