MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 641 



The same sequence of observations as above is repeated for the second set of 

 readings, with this difference, that the galvanometer is now connected with the 

 exploring coil which measures the longitudinal component of the induction. The 

 figures given under the column headed " total " might in this instance have shown both 

 the final positive and immediately previous negative reading. Instead of this, however, 

 the average of both these readings has simply been given. Column B, therefore, is the 

 average of the final values of the induction which is alternately positive and negative. 

 Owing to the large air circuit embraced by the exploring coil, the measured induction 

 is too large by about 7 per cent. This has not been corrected. The iron being 

 demagnetised at each increment of H c , the resulting induction produces no longitudinal 

 component, and no throw in the galvanometer was observed. .The effect of the 

 circular induction is, however, plainly shown when T^ is superposed, and the 

 readings then obtained ought to be compared with those in the fourth, sixth, and 

 eighth sets, which differ only in that higher values of the longitudinal field are 

 used, viz., H^= 1T9, 2T5, and 39*6 C.G.S. units respectively. 



§ 16. Table VII. gives the results of experiments when, upon the induction due 

 to four different values of the longitudinal field, the circular field, between the limits 

 of H c = and Hc=13 C.G.S. units, is first superposed (LC * conditions) and then 

 repeatedly reversed (LCC* conditions). During the first, third, fifth, and seventh sets 

 of readings, the galvanometer is connected with the exploring coil measuring circular 

 induction or the circular component of the induction, and during the second, fourth, 

 sixth, and eighth sets, with the exploring coil measuring the longitudinal induction or 

 the longitudinal component of the induction. The four different values of Hj are the 

 same as in Table VIII. , but the order in which the two fields are superposed is 

 reversed. After the readings for any single value of H,. have been completed, the 

 iron is demagnetised by decreasing reversals of the circular field. 



§ 17. The measurements given in Tables VII. and VIII. are plotted in figs, xi., 

 XII., and xin. for the values of H ; = 3, 21*5, and 39'6 C.G.S. units, omitting those 

 for H < =11*9 as superfluous owing to their intermediate character. In these three 

 figs, the dotted curves represent the normal induction curves when the circular force 

 and the longitudinal force are each acting alone. Necessarily the latter induction, 

 being due to a force kept at a constant value, is represented by straight lines parallel 

 to the horizontal axes. The other four curves, marked B c , measure the circular 

 components of the induction due to H c ; and the other four curves, marked B,, the 

 longitudinal componeut of the induction due to H^. In all cases the heavy full 

 lines measure the component of the induction in the direction of the force first 



* These letters refer to the circular and longitudinal fields respectively. The order in which they occur signifies 

 the order in which they are superposed the one upon the other. Thus " CL conditions" means that the circular 

 field H c acts first upon the iron, and the longitudinal field, second. " LC conditions " has, necessarily, the opposite 

 signification. CO and LL mean that the circular and longitudinal fields respectively are repeatedly reversed. § 38 

 may he referred to. The definitions there given are immediately applicable, by substituting L (longitudinal field) 

 tor T (transverse field). 



