562 HISTORY OF SCIENCE, 



if the wire held as shown be connected at the end F with the positive 

 pole (page 556), and at the end F' with the negative pole of a voltaic 

 pile or battery, an immediate effect is observed the moment these con- 

 nections are completed. The needle swings round in the direction 

 shown by the curved arrows at A and B, and finally settles in a position 

 in which it is transverse to the wire, partially or entirely according to 

 the strength of the current. The direction of this movement is always 

 the same under the same circumstances. If the wire, instead of being 

 held above the needle, be held below, as in Fig. 273, the direction of 

 the movement is inverse to the former case. If the wire be held 

 vertically instead of horizontally, deflection of the needle will also take 

 place ; and in all cases the deflected position into which the needle 

 settles after a few oscillations will be maintained so long as the current 

 continues to flow through the wire, kept in the same position. On 

 the cessation of the current, or the removal of the wire, the needle 

 reverts to its ordinary position in the magnetic' meridian. We see by 

 this experiment that the current flowing through a wire strangely modi- 

 fies the surrounding space, which acquires the new property of mag- 

 netic polarity. The action of a wire conveying a current is denned 



FIG. 274. 



and will be readily remembered by what is called Ampere's Rule. 

 Imagine that a man is swimming with the current, that is, in the direc- 

 tion (page 551) in which it flows, and that his face is towards the 

 needle, then the north pole of the needle will be deflected towards his left. 

 This is illustrated in Fig. 274, where the action of each of the currents 

 represented by the arrows would be to turn N, the north-seeking pole, 

 out of the plane of the paper towards the spectator. 



These details will at once make clear to the reader the meaning of 

 the word " current " as applied to electricity. He will understand that 

 every part of the circuit, including the pile or battery itself, has the 

 same effect on the magnetic needle, and that the notion of a current, 

 with its imaginary direction, is only a matter of convention. In the 

 conducting-wire which unites the poles of an active voltaic pile the 

 electric "fluids" are conceived to be constantly flowing, and the 

 phenomena to which the current gives rise are said to be those of 

 dynamic electricity. The opposite condition, namely, that in which 

 the " fluids " are not flowing, is static electricity. 



