NATURAL PIIILOSOPnY. 161 



MAGNETO-ELECTRIC MACHINES. 



In Mr. "Wilde's machine, the induced current from permanent 

 magnets is made to excite a row of electro-magnets, so that the 

 armature of these secondary magnets gives off a powerful current 

 of electricity, which may be at once applied to experimental 

 purposes, or made to excite a third row of electro-magnets, and 

 so ad infinitum. In the machine of Messrs. Siemens and Wheat- 

 stoue, the trace of residual magnetism in a soft iron electn)- 

 magnet is made to act upon the armature, and the wires from this 

 armature, being carried around the electro-magnet, increase the 

 intensity of the magnetism already present there ; they, in turn, 

 increase tlie power of the current from the armature, so that, by 

 constantly turning the handle, the quantity of electricity increases 

 till a powerful current is obtained. In the machine invented by 

 one of Mr. Ladd's assistants, the electro-magnet is fitted with a 

 double armature. The current from one is used to increase the 

 power of the electro-magnet, and that from the second is the one 

 used for experimental purposes. This machine, when turned by 

 hand, will heat 7 inches of platinum wire .01 of an inch thick to 

 redness ; and will give flashing sparks of the electro-light between 

 carbon ])()ints, and would give a continuous illumination if turned 

 by machinery. 



"At the 1868 meeting of the British Association, Mr. W. Ladd 

 made the following communication " on a further development 

 of the Dynamo-Magneto-Electric Machine." 



" At the meeting of the British Association, last year, I brought 

 before the section one of m}^ small dynamo-magneto machines, 

 the first that had been made upon that principle. I have since 

 constructed a much larger machine, and it may be interesting now 

 to give some jjarticulars respecting it. The object in constructing 

 it was to sujiply a good electric light for the purpose of lecture 

 demonstrations. It is constructed upon the double-armature prin- 

 ciple, both armatures being placed end to end, so that their mag- 

 netic axes cross each other at right angles. The short armature 

 contains 108 feet of very stout copper wire, and sends its currents 

 into 240 lbs. of copper wire surrounding the electro-magnet, ex- 

 citing a large amount of magnetism in the body of the machine. 

 And as the second armature is also made to revolve between the 

 poles of this electro-magnet, a sufTicient effect is produced at the 

 two ends of the 312 feet of very stout copper wire (which is wound 

 upon it) to produce a good electric light from the carbon poles of 

 the regulator. But in order to make that light sufficiently con- 

 tinuous, it is requisite that the armatures should revolve from 

 1,800 to 2,000 revolutions per minute ; but as the armatures have 

 to be magnetized and demagnetized twice during each revolution, 

 there would be in the latter caso 4,000 flashes of light per minute. 

 Now, it has been shown that every time iron becomes magnetized 

 it is elongated, and again shortened when demagnetized. At every 

 alteration, therefore, of the condition of the iron some small 

 amount of heat nmst be evolved, and would increase to such aa 

 14* 



