242 Lord Rayleigh on the Behaviour of Iron and Steel 



iron wire 3*3 millim. diameter. The insulated copper wire was 

 in three layers, of resistance *34 ohm, and the total weight of the 

 electromagnet was 283 grams. It was held so as to embrace 

 the upper needle system. When the time of swing from rest 

 to rest was 4 seconds, the movement due to a current of about 

 §o^oo ampere was 100 divisions. The zero was steady enough 

 to allow a displacement of half a division to be detected with 

 tolerable certainty in each trial ; so that, as actually used, the 

 arrangement was sensitive to a current of J x 10 -6 ampere. 

 The addition of a similar electromagnet embracing the lower 

 needle system, and connected in series, would double the 

 sensitiveness, and raise the resistance to "68 ohm. A galva- 

 nometer thus constructed, and of resistance equal to 1 ohm, 

 would show a current of 10~ 7 ampere. Using finer wire, we 

 might expect an instrument of 100 ohms to show a current of 

 10~ 8 ampere, and so on. 



For comparison with the above I tried, in as nearly as 

 possible the same way, the sensitiveness of a good Thomson 

 astatic galvanometer of resistance 1*3 ohm. With an equal 

 time of vibration, a current of 2q^q ampere produced a move- 

 ment of 300 divisions. The zero was perhaps a little steadier 

 than before ; but it will be seen that the sensitiveness was of 

 the same order of magnitude. In both cases, by taking pre- 

 cautions and by using repetition, the delicacy might have been 

 increased, probably tenfold. 



The experiments show that there is no difficulty in con- 

 structing a galvanometer of high sensitiveness upon these 

 lines. According to theory, with ideal iron of permeability 

 100, it should be possible to attain a much higher degree of 

 sensitiveness than without iron. But the tendency to retain 

 residual magnetism would certainly be troublesome, and pro- 

 bably neutralize in practice most of the advantage arising 

 from the higher permeability, which allows of windings more 

 distant from the needles being turned to good account. Another 

 inconvenience may be mentioned. If the iron poles are 

 brought at all close to the needles, there is a strong tendency 

 to instability at moderate angles of displacement. 



Experiments already described proved conclusively that the 

 response of iron and steel to small periodic magnetic forces is 

 not affected by the presence of a constant force, or of a re- 

 sidual magnetization, of moderate intensity. At the same time 

 it appeared in the highest degree probable that the indepen- 

 dence was not absolute, and that the response to a given small 

 change of force would fall off as the condition of " saturation" 

 is approached, even though we admit, in accordance with 



