II. 



ELECTRICAL EHEOMETRY. 



49 



of the new position. We have also given the numbers obtained from the experi- 

 ments opposite to those given by observation, to render comparisons easy. We 

 added at the end a comparison of the results belonging to the polar diameter of 

 the fflobe. 



Position of 

 the needle. 



Series 4th. 



Series of 1. 



Mean 

 of 3tl. 



Series No. 5. 



Globe. 





d = SO° 



d = 25° 



d = 17° 36' 



d = 30° 





Zc = 0.0699102 



Zc = 0.0694633 



Zc = 0.0696869 









cale. 



observ. 



calc. 



observ. 



calc. 



observ. 



calc. 



ob.serv. 



4 tenths. 



1.396 



1.155 



1.09824 



1.1315 



1.08817 



1.144 





1.158 



6 " 



1.40235 



1.481 



1.40694 



1.382 



1.48564 



1.367 





1.411 



7 " 



1.67597 



1.774 



1.69961 



1.648 



1.67551 



1.609 





1.642 



8 " 



2.23241 



2.270 



2.24484 



2.1815 



2.19097 



2.289 





2.093 



9 " 



3.41917 



3.687 



3.58416 



3.675 



3.8232 



4.193 



3.261 



3.283 



out 13 













— 



0.545624 



0.656 



This table shows that there is an agreement between the calculation and 

 observation ; the difference is at most the limits of errors probably committed in 

 the observation, except in the positions nearer to the circumference, where the 

 results of the experiments are always greater than those of calculation. The 

 figure (9) shows the increase of intensity of forces from the centre to the circum- 

 ference for a deviation of 30°. 



We shall now proceed to take into consideration, very briefly, the result of calcu- 

 lation and experiment for some positions of the needle within the globe. The results 

 belonging to the polar axis are given in the last two columns of the preceding table. 

 We can observe that for the globe the numbers given by observation are smaller at 

 y'ijths than those by calculation, but the reason is very apparent since we have seen 

 that the action of two currents making equal angles at both sides of the meridian 

 is not exactly the same as that of two equal currents disposed in the meridian 

 plane, but must be a little less. Thus, if we suppose two circular wires inclined 

 30° on each side of the magnetic meridian, and the needle to deviate 30° likewise 

 from it, we obtain for the ratio of forces between the centre and T^ths, from one 

 circle 4.737, and from the other 2.196, for the values of the function Z in formula 

 (7), § 6, from which we conclude the ratio of forces = 3.261, a little smaller than 

 with two parallel wires. What is said at the end of § 8 cannot therefore be 

 extended to 30° of deviation. Besides this, table A, § 10, shows that a little 

 variation of place near the pole produces a great difference in the result, and that 

 such experiments must never be tried again with thick coils of wires. 



As for the action of the globe on its other diameters, no exact coincidence may 

 be expected between the formula and observation, since the divergence of wires 

 has a sensible influence on it. The difference must evidently be greater along the 

 13 



