231 



scale (which was at the distance of eighteen inches from the 

 mirror) was 3'. 98. The diflferences of the observed and 

 calculated results, therefore, do not in general exceed the 

 amount which may be fairly ascribed to errors of observa- 

 tion ; and the accordance is sufficient to establish the fact, 

 that the changes of the induced force of the bar are, within 

 the observed limits, proportional to those of the inducing 

 action. It is important to observe also that the changes of 

 the induced force, produced artificially in these experiments, 

 are much greater than any which are likely to arise from the 

 variations of the vertical component of the earth's magnetic 

 force, and therefore that the experiments may be regarded 

 as severe tests of the performance of the instrument. 



The preceding observations further showed, that the 

 changes in the inducing force were instantly followed by their 

 eifects upon the suspended magnet ; so that the changes of 

 induced force required no appreciable time for their develop- 

 ment. It remained only to ascertain, in a somewhat fuller 

 manner, how far the bar was susceptible of minute magnetic 

 changes, from very small variations of the acting force. For 

 this purpose, a series of readings of the scale was taken, the 

 inclination of the bar to the vertical being altered by half a 

 degree between the consecutive readings. The mean diffe- 

 rence of the successive readings was found to agree, very 

 exactly, with the calculated difference; while the jooriio/ dif- 

 ferences deviated from the mean by an amount not exceeding 

 the limits of error of observation. It may be presumed there- 

 fore, that the changes of the induced force in the iron bar 

 are continuous; and, accordingly, that the sensibility of the 

 instrument is only limited by the optical power, which is ap- 

 plied to observe the changes of position of the suspended 

 magnet.* In the experiments above described, the arc-value 



* Against this conclusion is the fact, that considerable changes in the induced 

 force of the bar seem to be attended with some jtermanenl changes of polarity ; 

 and it may be presumed that the same thing will take place, in a proportionate 



