Miscellaneous. 



301 



ness. In order to ascertain the law of electro-magnets of different lengths, 

 M. Lenz and I undertook numerous and laborious observations, which were 

 extended even to rods of thirteen feet in length, and keeping in view at the 

 same time the determination of the particular distribution of magnetism 

 in the rods. Among these observations I shall only refer to such as seem 

 most applicable to electro-magnetic machines, and which have yielded re- 

 sults as simple as unexpected. The following table contains the results 

 of some observations made with rods of the same diameter, but of dif- 

 ferent lengths, covered with electro-magnetic helices, and influenced by 



a current of the same force. M being the magnetism of the extremities, 



M 



and n the number of the coils of the helix, we have - = x, a formula 



n 



according to which we may calculate the numbers contained in the 

 third column. The numbers in the fourth column are deduced from a 

 series of other observations, made with the same helix of 960 turns, 

 which did not cover the whole length of the rods, but were collected at 

 the extremities only, where they occupied a space of about two inches in 

 length. The helices being the same in all the observations, it was only 

 necessary to divide the magnetism of the extremities by 960, in order 

 to find the numbers of this column. 



Table of Experiments upon the Magnetic Forces of Rods of different lenyths. 



Length of 

 the rods. 



3' 



2'.5 



2 



1.5 

 1 



0.5 



Number of 

 Coils. 



946 

 789 

 634 

 474 

 315 

 163 



Mean Value of One 

 Coil, if the Helix 

 occupies the whole 

 length. 



7,334 

 6,993 

 7,402 

 7,880 

 7,847 

 7,766 



7,537 



iVJean Value ol 

 One Coil, if the 

 Helix occupies 

 only the ex- 

 tremities. 



7,560 



7,264 



6,871 



7,491 



7,573 



7,691 



7,408 



" From these numbers, it will be seen that the influence of one coil of 

 the helix is nearly the same for all the rods, and that their length does 

 not exercise any specific influence. It is only in proportion to the 

 number of the turns or revolutions, and to the force of the current, that 

 the rods can acquire a greater or less amount of magnetism. The small 

 rods even appear to have a slight advantage over large rods, since it has 

 been found by experiments that the actual force of rods of three feet, 

 bears to that of rods of half a foot the ratio of seventy- three to seventy - 

 seven. It is also found, that there is a gain of seventy-five to seventy- 

 four when the whole length of the rods is covered, instead of simply 



2 R 



