to Electricity and Magnetism. 509 



needle is first affected by the action of A B, it indicates a cur- 

 rent in the adverse direction to the secondary current. 



80. Although the two inductions in the tertiary conductor 

 nearly neutralize each other, in reference to the indications ol 

 the galvanometer, yet this is far from being the case with re- 

 gard to the shocks, and the magnetization of steel needles. 

 These effects may be considered as the results alone of the 

 action of A B ; the induction of B C being too feeble in inten- 

 sity to produce a tertiary current of sufficient power to pene- 

 trate the body, or overcome the coercive power of the hard- 

 ened steel. Hence, in reference to the shock and magneti- 

 zation of the steel needle, we may entirely neglect the action 

 of B C, and consider the tertiary excitement as a single cur- 

 rent, produced by the action of A B ; and, because this is the 

 beginning induction [56.\ the tertiary current must be in an 

 opposite direction to the secondary. For a similar reason, a 

 current of the third order should produce in effect a single 

 current of the fourth order, in a direction opposite to that of 

 the current which produced it, and so on : we have here, there- 

 fore, a simple explanation of the extraordinary phtenomenon 

 of the alternation of the directions of the currents, of the dif- 

 ferent orders, as given in this and the preceding paper. See 

 paragraph 25. 



81. The operation of the interposed plate (32, 47,48, &c.), 

 in neutralizing the shock, and not affecting the galvanometer, 

 can also be readily referred to the same principles. It is cer- 

 tain that an induced current is produced in the plate (III. 64.), 

 and that this must react on the secondary in the helix; but 

 it should not alter the total amount of this current, since, for 

 example, at the ending induction, the same quantity of cur- 

 rent is added to the helix, while the current in the plate is 

 decreasing, as is subtracted while the same current is increas- 

 ing. To make this more clear, let the inductive actions of 

 the interposed current be represented by the parts of the 

 curve, fig. 20. The induction represented by A B will re- 

 act on the current in the helix, and diminish its quantity, by 

 an amount represented by the ordinate bV>; but the induc- 

 tion represented by B C will act, in the next moment, on the 

 same current, and increase its quantity by an ec|ual amount, 

 as represented by the same ordinate BZ»; and since both ac- 

 tions take place within a small part of the time of a single 

 swing of the needle, the whole deflection will not be altered, 

 and consequently, as Ihr as the galvanometer is concerned, 

 the interposition of the plate will have no perceptible effect. 



82. But the action of the plate on the shock, and on the 

 magnetization of tempered steel, should be very ilirtiirent; 



