PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 393 



.A'eiw subject. — On motion of Mr. Fisher, 



The subject of " Gunboats" was selected for discussion upon 

 the next evening. 



American Institute, Polytechnic Association, ? 



June 20, 1861. ^ 

 Prof. Cyrus Mason in the chair, 



NEW electro-magnet. 



Dr. Vanderweyde exhibited and explained an electro-magnet 

 upon a new principle. The magnetic action of a wire conduct- 

 ing a voltaic current extends in all directions in a plane perpen- 

 dicular to its axis. If, therefore, a spiral wire surrounds a core 

 of iron, the magnetic action of the wire is only made available 

 on one side of the wire, that of the other side being lost. In 

 1852, Nickle constructed an electrp-magnet, having a bar on each 

 side outside of the coil ; both of these were connected Avith one 

 end of the core. This electro-magnet was found to be stronger 

 than one formed of the same quantity of iron and coil used in the 

 common way. The effect of the outer bars was to render a por- 

 tion of the magnetic action of the wire opposite the central core 

 available. Upon a similar principle, Roberts constructed an 

 electro-magnet, made of a block of iron about seven inches 

 square, with deep grooves, the copper Avire being wound around 

 the portions of the iron thus separated, so that these portions 

 are ultimately north and south poles. Witli this magnet, he 

 suspended 2,676 pounds. But here the portions of the Avire 

 outside of the magnet have not the Avhole of their magnetic 

 action brought into play. The new arrangement proposes to 

 extend this principle so as to use all the magnetic poAver of the 

 wire, by surrounding it Avith the iron. There is an interior iron 

 core, Avound Avith a helix, outside of Avhich is an iron tube con- 

 nected Avitli the core at one end by an iron disk through Avhich 

 the extremities of the Avire pass to the battery. The keeper is 

 a disk placed upon the other end, fitting exactly both the core 

 and the tube. The polarity^ of the surrounding tube Avill be the 

 reverse of that of the central core ; so that one pole Avill be in 

 the center, and the other around the circumference. If the iron 

 core Avere removed from this electro-magnet, it Avould have very 

 little poAver, the action of the Avires on the opposite sides neu- 

 tralizing each other's effect; but the introduction of the core, 



