Galvanometers of High Sensibility. 257 



Saturation of Magnets. — The experiments with magnet B 1 

 (see preceding table), magnetized in fields of 2200 and 5200 

 lines, and with magnet B 2 , magnetized in fields varying from 

 800 to 3400 lines per sq. cm., show that a limit to the intensity 

 of magnetization attainable is soon reached, practically in a 

 field of 1000 lines of induction per cm 2 . We undoubtedly 

 have the isthmus effect coming in when these small magnets 

 are magnetized by placing them between the faces of an elec- 

 tromagnet ; thus in B, which was magnetized in a field (undis- 

 turbed) of 2200, an average intensity of magnetization of 284 

 is reached, which must correspond to an average remanent 

 induction of at least 3500. 



Effect of Length and Cross Section of Magnets. — The experi- 

 ments made on the group of fine hair-spring magnets H x to 

 H 7 , all taken from one piece tempered glass hard in water, 

 shows the variation of the magnetic moment with the length. 

 These results are plotted in a curve, fig. 6 (p. 260), which shows 

 that, for short magnets, the magnetic moment decreases much 

 more rapidly than in proportion to the length. 



The effect of cross section on the intensity of magnetization 

 is shown by comparing, e. g., H 6 and B 3 , the average intensity 

 of magnetization in H 6 (although shorter) being more than 2*7 

 times that in B 3 . Similarity A 5 (width reduced by grinding), 

 although shorter, has a considerably greater intensity of mag- 

 netization than A 3 . This strongly emphasizes the fact that 

 probably the greatest gain in sensibility at present at our dis- 

 posal is in the use of a number of magnets of small cross sec- 

 tion instead of one (or so) of large section, i. e., if you have a 

 given mass of magnetic material (to be made up of magnets 

 of a given length), the system will be considerably more sen- 

 sitive if you use a number of magnets of small cross section. 

 This brings up at once the question whether, when you divide 

 up the magnetic material into a number of thin magnets, you 

 do not in the end lose what you gained in magnetic moment 

 by subdivision, by the fact that in galvanometer systems the 

 magnets must be placed very near together and so tend by 

 their mutual action to weaken one another. 



Mutual Action of Magnets. — This question was investigated 

 for many sizes of magnets and under various conditions ; one 

 or two experiments will, however, be sufficient to show that the 

 effect is much smaller than is usually supposed. To study this 

 the magnets B : were fastened to one of the brass inertia discs, 

 magnetized, and period determined, in an exhausted receiver, 

 when the distance between the centers of the magnets was 

 3*3 mm ; the magnets were then pushed together until the dis- 

 tance between the centers of the magnets was l*2 mm , when the 



