133 Electro-Magnetic Rotations. 



magnet, and which are fixed to the axis, one above and the other 

 below the magnets. These pole changers are fixed so that each 

 magnet changes its pole when at right angles to the coil of ribbon. 

 A strip of copper G is soldered to the copper F, and the outer ex- 

 tremity of the ribbon C D, while the inner extremity of the 

 ribbon is connected with another strip similar to F upon the other 

 side of the apparatus. Upon connecting these two strips of cop- 

 per with the two poles of a battery, the magnets will revolve with 

 rapidity. This motion is much increased if a galvanic magnet 

 H I be laid across the coil and at right angles to it, so that its poles 

 are near the poles of the revolving magnets when their poles are 

 changed. The same change of poles answers for both, and we 

 have in this way the combined effect of a rotation produced both 

 by a double magnetic force, and the tangential action of a current. 

 All persons who have made experiments in electro-magnetism 

 must have observed, that large magnets cannot be made to revolve 

 with as great rapidity as small ones. The small magnet will 

 not only make the greatest number of revolutions, but its ex- 

 tremities will move with much greater rapidity than the extrem- 

 ities of the large magnet, notwithstanding the latter a much 

 greater circle. This is owing to several causes, one of which 

 may be a deficiency in the quantity of the galvanic fluid, there 

 not being a sufficient quantity generated in a small battery to 

 saturate a large magnet, at least for more than a few moments 

 after the commencement of its action. But when there is bat- 

 tery power sufficient, there is often deficiency in the means of 

 communication of the electrical current to the rotating magnet. 

 The communication being by means of two wires or small strips 

 of metal, and no greater with a large magnet or circle of mag- 

 nets, than with one small magnet, and as the most finished polish- 

 ing of the wires, and tinning of the semicircles of the pole chan- 

 ger, can bring little more than one point of each into contact, the 

 whole supply must pass through these two points. The current 

 on this account does not pass as freely as it ought, and the mag- 

 nets do not possess the power they would if the communication 

 was less impeded. That this is the case, may be proved by 

 pressing the communicating wires of a revolving magnet together, 

 and the power of the magnet will, to a certain extent, increase 

 nearly with the pressure. But in a revolving circle, if we increase 

 the pressure beyond a certain degree, the increase of friction im- 



