proceedings of the polytechnic association. 347 

 Great Electro-Magnet. 



Tlie Cliainnan illustrated on the blackboard this magnet of which the 

 following' is a description: 



The engraving represents a large magnet constructed for the Free 

 Academy of this city, by Charles T. and J. W. Chester. Connected with 

 it are attachments for exhibiting diarnagnetic effects, but the magnet 

 itself can be easily removed from these attachments, ancf its moveable base, 

 and can be suspended by a ring, so aa to exhibit its enormous lifting 

 capacity. The magnet is built in the same manner as small electro- 

 magnets, by securing straight round bars into a cross flat bar, and if con- 

 structed by simply bending a round bar into the horse-shoe form, it would 

 measure five feet in length. The core is of the purest soft iron four inches 

 in diameter, and is covered with a winding of insulated copper wire, eight 

 number thirteen wires, being carefully wound on at once. Two hundred 

 ])ounds were used to complete tlie magnet. The armature weighs 80 

 pounds, and no weight applied to this, thus far, has been able to detach it 

 during a charge of the magnet. 



In the engraving, the magnet is placed upright, and two blocks of iron 

 (a. a.) are placed upon the magnet faces, and adjusted in position by the 

 screws (n. i>.) so that the magnet poles can at pleasure be placed at any 

 required separation, and assume either a flat or pointed shape. The turn 

 table (c.) is made to drive the copper tube (e.) with great speed. By dia^ 

 magnetic laws, copper placed between the magnetic poles, during a strong 

 magnetic charge, resists motion and acts as if placed in a demi medium. 



The copper tube (e.), rotating freely while the magnet is inactive, becomes 

 suddenly fixed upon charging the magnet, and if in defiance to the resistance 

 to motion thus occasioned, sufficient force is applied to rotate the tube it 

 becomes gradually warmer, and finally so hot aa to boil water and blow 

 out a cork used tc close the end of the tube. To demonstrate this dia- 

 rnagnetic influence, in another way, the tube is now removed and the 

 copper disk, shown just behind it, is brought into the same magnetic 

 influence. By the wheels (f.) the cord suspending the pyramidal system 

 of mirrors 13 is twisted, and recovering itself from this torsion imparts 

 rotation to 13 and at the same time to the copper disk. A beam of light 

 thrown upon the mirrors is, during their rapid rotation, reflected upon the 

 walls of the lecture room in an apparent circle. But the instant that the 

 magnetic influence arrests the rotation of the copper disk, the mirror stops 

 as if served by the hand, and the arc of light is as suddenly converted 

 into a stationary spot. 



Chester's Electropoin Battery. 



This battery consists of gas carbon, artificially prepared, in rods, and 

 connected with platina wires or platina buttons, and zinc, for the metalic 

 elements. Chromic acid and sulphuric acid dilute, compose the fluid ele- 

 ments. It was introduced by Charles T. and J. N. Chester, first experi* 

 mentally, in Canada, January, 1861, and afterwards in this country, where 

 it has been almost universally adopted. About 3,000 cups are in use in 

 this city for telegraph purposes, at a saving of at least one-half the price 



