126 Magneto-Electricity, and Electro-magnetical Machines. 



6. Electricity thus produced by a magnet, is called magneto- 

 electricity. 



7. The common magneto-electric machine consists of a wrap- 

 ped "keeper," revolving almost in contact with the poles of a 

 powerful horse-shoe magnet. This form of the instrument is 

 attributed by Mr. Faraday to Mr. Saxton, now residing in Phila- 

 delphia. For figures and descriptions, the reader is referred to 

 the last number of this Journal. 



Having premised these elements, I proceed to the subject of 

 my investigation, which was to determine whether more electri- 

 city is developed in a coil by passing the included iron abruptly 

 hy the pole of a magnet, or by passing it along from the middle 

 or neutral point of the magnet to the pole, as close as possible to 

 it throughout the whole course, thus exciting the polarity grad- 

 ually. As I solved the problem with a new instrument which an- 

 swers several other purposes, I will first describe that instrument 

 and the several uses which I have made of it. I propose to call it 

 the Electro-magnetic Dipping-needle. It was in the first place 

 intended only for class experiments, and consisted of an iron bar 

 eleven inches long, half an inch wide, and one tenth of an inch 

 thick, bound with about twenty five feet of copper fillet, and fast- 

 ened to a horizontal axis about two and a half inches long, piv- 

 otted in two upright brass columns so as to give rotary motion in 

 the plane of the meridian, exactly like the motion of the dipping- 

 needle. On the axis were two copper wheels about half an inch 

 in diameter, insulated, and running in mercury grooves in a piece 

 of ivory, and having the two ends of the copper fillet soldered to 

 them. By connecting the poles of the battery with the mercury 

 grooves it became an electro-magnet, having free rotary motion. 



Class experiment of showing the Dip hy Electro-magnetism. — 

 The needle being placed horizontally, and the poles of the bat- 

 tery connected with the mercury grooves so as to produce polarity, 

 that end possessing north polarity immediately descended to the 

 line of the dip. The wires being changed so as to reverse the 

 polarity by reversing the current of electricity, the opposite end 

 immediately descended to the same line. Thus to exhibit stri- 

 kingly the effect of terrestrial magnetism in producing the dip, 

 was all that had been so far contemplated. 



The Dipping-needle made to revolve by Terrestrial Magnet^ 

 ism. — I afterwards attached to the axis four semicircular "cams," 



