184 Dr. Tyndall on the Progress of the Physical Sciences. 



weakened by the N-form and strengthened by the U-forra thereof; 

 and the same is true of currents of all other orders. 



With regard to the directions of these currents much uncer- 

 tainty exists ; Henry, Matteucci, Verdet and Knochenhauer have 

 given utterance to various and contradictory opinions on this 

 subject. With admirable ingenuity M. Riess has brought the 

 foregoing experiments to bear upon this point. Let us suppose 

 an induction-disc to be placed in the primary circuit, and 

 parallel to it another with its ends united, thus forming an iso- 

 lated circuit in itself. The passage of a current through the 

 former will arouse an induced current possessing a certain direc- 

 tion, either opposed to the primary or coincident with it, in the 

 latter. Without altering the relative position of the discs, let 

 the second one be conceived to be brought into the primary 

 circuit ; the current, in passing through the first, will, as in the 

 former case, induce a current in the second; but now primary 

 and secondary are in the same wire, and it evidently depends 

 upon the manner in which the two discs are connected with each 

 other whether both currents meet in opposition* or flow on in 

 the same direction. If by the N-combination secondary and 

 primary flow on together, then by the U-combination they will 

 oppose each other, and vice versa. Now the constant weakening 

 effect of the N-form, and strengthening effect of the U-form in 

 currents of all orders, demonstrate a constant relation between 

 the directions of the induced and inducing currents. If the 

 relation of any one secondary to its primaryf* with respect to 

 direction, be determined, the same relation holds good in all 

 other cases ; if the currents have the same direction in one case, 

 they will have it in all cases ; if opposed once, they are opposed 

 throughout the entire sei'ies. Commencing at tbe current which 

 passes direct from the battery, it is easy to see that whatever be 

 the direction of the secondary which it arouses, the tertiary 

 evoked by the secondary must necessarily have the same direc- 

 tion as the current passing from the battery ; for if the second- 

 ary be opposed to the primary, the tertiary will be opposed to 

 the secondary, and hence have the same direction as the pri- 

 mary ; and if the secondary have the same direction as the pri- 

 mary, the whole series will have this direction. Thus we arrive 

 at the following necessary conclusion : — Currents of the third, 

 fifth, and other odd orders, have the same direction as the original 



* We must guard ourselves here against the notion that the opposition 

 of primary and secondary is in any degree similar to the mechanical oppo- 

 sition of two forces, or even to the opposition of two currents of the same 

 order. 



t The terms secondary and primary are used here in a relative, not in 

 an ahsolute sense. 



