Magneto-Electricity, mid Electro-magnetical Machines. 127 



running in mercury grooves in such a manner that the needle, by 

 its own motion, produced the necessary reversals, when, upon the 

 application of a vigorous calorimotor, it performed one hundred 

 and fifty revolutions per minute, in the plane of the magnetical 

 meridian, thus exhibiting terrestrial .magnetism in a very agreea- 

 ble manner. 



The 7iorth end of the eojrth shown to he virtually a Magnetical 

 south pole. — While the needle was revolving by terrestrial mag- 

 netism, I brought the south pole of a feeble artificial magnet to 

 the lower point of the dip, so that the pole of the needle passed 

 near to it. The motion was immediately accelerated. On pre- 

 senting a north pole at the same point the motion was retarded, 

 stopped, or reversed, according to the strength or proximity of 

 that pole. 



The south polarity of the north end of the earth still more stri- 

 kingly exhibited. — I constructed a semicircular steel magnet, the 

 inside diameter of which just permitted the dipping-needle to re- 

 volve within it, and attached it in such a manner, that the south 

 pole of it was at the lower point of the dip, and bending round to 

 the south had its north pole at the upper end of the dipping axis. 

 The battery being applied, the revolutions were exceedingly rapid, 

 and in the same direction as hy terrestrial magnetism. On re- 

 versing the semicircular magnet and bringing the 7iorth pole at 

 the lower point, the motion was reversed, and contrary to that 

 produced by terrestrial magnetism. In this form, the instrument 

 resembles Messrs. Davenport and Cooke's model, as exhibited last 

 spring in New York, one of their semicircular magnets being re- 

 moved and the instrument being turned down on one side, so as 

 to bring the diameter of the other into the dipping axis. 



Magneto- Electricity produced hy Terrestrial Magnetism. — 

 Removing the semicircular magnet and the battery, connecting 

 the poles of my thermoscopic galvanometer with the mercury 

 grooves, and giving the needle a smart whirl by hand, say one 

 hundred and fifty revolutions per minute, I obtained a sufficient 

 quantity of electricity to deflect the galvanometer needle 40° by 

 impulse, and that too against a torsion Avire six inches long, 

 weighing one third of a grain. And here I ought to remark, that 

 the mechanism used for the reversal of electrical currents, and 

 consequently of the polarity, when the instrument is used as a 

 self-revolving machine, was precisely what was required in pro- 



