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. or 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 
by 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 
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 by 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,” 
