182 Transactions of the American Institute. 



But if " the earth itself is a great magnet," it also must affect sur- 

 rounding space — stretching thousands of miles from its surface — 

 with this magnetic influence. The dipping needle has shown us its 

 lines of force. These lines, stretched above this stage, show as 

 truly the direction in which its magnetism acts, as this plumb-line 

 shows the direction in which gravity acts. If a dipping needle were 

 carried all over the area of this city it would be found everywhere 

 to point parallel to these lines. Let your imagination lead your 

 minds along their paths into their heights above us, just as we have 

 seen the long streamers of the polar lights tracking them out for us. 



But if all this be so, then why can we not evolve a current of 

 electricity from the earth's magnetism, as we did from yonder magnet ? 

 That, indeed, would be a grand experiment ? Let us hasten to try 

 it. Here I have a coil of 300 feet of copper wire wrapped in a circle 

 of 2£ feet in diameter. I place it thus, so that the earth's magnetic 

 lines pass through its circle. You observe that its plane is at right 

 angles to the direction of these stretched cords. The conditions, 

 therefore, are absolutely those which we had when experimenting 

 with the electro-magnet. Now, how will I cut these terrestrial lines? 

 By rotating the ring around a north and south axis, just as we rotated 

 this same coil around a vertical axis, when we obtained an electric 

 current from the large magnet. The ends of the coil are connected 

 with the galvanometer, and now look at the needle on the screen 

 while I turn this ring. See, it moves ; a current of electricity has 

 passed round the coil and out of it into the galvanometer.* Consider 

 the simplicity of the apparatus which has given us the astonishing 

 result. Nothing but a coil of wire and a galvanometer. No battery 

 used, as in the experiments which explained the galvanometer to 

 you. No magnet near, as when we got a current from that one. 

 Yes, no magnet near — but the earth. For now I can boldly say, 

 " the earth itself is a great magnet," and not only a great magnet, 

 but a strong magnet. Let us put it against our magnet. How far 

 will we be obliged to remove ourselves from it to obtain the exact 

 degree of deflection in that needle which' the earth has just given % 

 Then we can compare them ; for, at the distance of so many feet from 

 the poles of our electro-magnet (the coil being, in reference to its 

 poles, similarly placed with reference to the earth's), the force it sets 

 in motion in this coil exactly equals the force which the earth, at 

 some thousounds of miles from its poles, sets in motion in the same 

 coil. Let us make the calculation. Suppose we find from experi- 



* Farady, Exp. Resear. in Electr. 



