Company of Milan furnished us with a remarkably soft iron, the 

 ' Armco' iron of the Armco Company (Genoa). The permanent field of 

 this iron is so weak that an air-space of 2-^ô' i^- would have sufficed for 

 the permanent magnetism of our magnet to be diminished to an in- 

 significant value, after the current was interrupted. However, as minor 

 deformations are always to be feared, we chose a distance between 

 armature and magnet of jyj in., which makes a total air-space of 5^- in. 



I 



Fig. 19. Electro-magnet holding a ballast tub 



To obtain this air-space we placed in sockets made in the upper part 

 of the cylinder six steel balls /^ of y in. diameter, which stand out 

 exactly yyj in. When the magnet is energized with the lever discon- 

 nected from the tubs each of these balls has a load of 726 lb. So that 

 they will not penetrate the mild steel, small washers of hardened steel 

 are placed above and below the balls. Thus the contact between the 

 armature and the magnet only occurs on six areas of very reduced 

 dimensions. This was necessary to avoid all danger from suction. 

 Before each dive, one checks, by means of a feeler gauge, whether the 

 air-space is still large enough. 



Each magnet consumes 36 watts. The attractive force which it 

 exercises upon the armature is 4400 lb. As the mechanical advantage of 



[•59] 



