330 EARTH MAGNETIC BY INDUCTION. SECT. XXXV. 



rents. When a hollow helix of copper wire, whose 

 extremities are connected with the galvanometer, is 

 placed in the magnetic dip, and suddenly inverted sev- 

 eral times, accommodating the motion to the oscillations 

 of the needle, the latter is soon made to vibrate through 

 an arc of 80 or 90. Hence it is evident, that what- 

 ever may be the cause of terrestrial magnetism, it pro- 

 duces currents of electricity by its direct inductive power 

 upon a metal not capable of exhibiting any of the ordi- 

 nary magnetic properties. The action on the galvanom- 

 eter is much greater when a cylinder of soft iron is 

 inserted into the helix, and the same results follow the 

 simple introduction of the iron cylinder into, or removal 

 out of, the helix. These effects arise from the iron 

 being made a temporary magnet by the inductive action 

 of terrestrial magnetism ; for a piece of iron, such as a 

 poker, becomes a magnet for the time, when placed in 

 the line of the magnetic dip. 



M. Biot has formed a theory of terrestrial magnetism 

 upon the observations of M. de Humboldt as data. As- 

 suming that the action of two opposite magnetic poles 

 of the earth upon any point is inversely as the squares 

 of the distances, he obtains a general expression for the 

 direction of the magnetic needle, depending upon the 

 distance between the north and south magnetic poles ; 

 so that if one of these quantities varies, the correspond- 1 

 ing variation of the other will be known. By making 

 the distance between the poles vary, and comparing the 

 resulting direction of the needle with the observations 

 of M. de Humboldt, he found that the nearer the poles 

 are supposed to approach to one another, the more the 

 computed and observed results agree ; and when the 

 poles were assumed to coincide, or nearly so, the differ- 

 ence between theory and observation is the least possi- 

 ble. It is evident, therefore, that the earth does not 

 act as if it were a permanently magnetic body, the dis- 

 tinguishing characteristic of which is, to have two poles 

 at a distance from one another. Mr. Barlow has inves- 

 tigated this subject with much skill and success. He 

 first proved that the magnetic power of an iron sphere 

 resides in its surface ; he then inquired what the super- 

 ficial action of an iron sphere in a state of transient mag- 



