86 Electro-Magnetic Apparatus. 



Art. X. — Electro-Magnetic Apparatus ; by Benjamin F. Joslin, 

 M. D. Prof, of Nat. Phil, in Union College. 



Professor Silliman. — Sir — In the construction of powerful 

 electro-magnets, the spiral wire, with various modifications, has been 

 hitherto universally employed for transmitting the magnetizing agent. 

 I have recently taken a different route, with a success equal to my 

 expectations. My method consists in applying to a bar of iron a 

 single rectangular sheet of copper, of a width nearly equal to the 

 length of the bar. The silk is simply laid on the copper and both 

 wound simultaneously around the iron, in a direction exactly trans- 

 verse. 



A piece of gun barrel eight inches and one fourth in length, about 

 three fourths of an inch exterior diameter, and varying in thickness 

 from one sixteenth to one eight of an inch, and weighing five ounces 

 and one seventh, was covered in this manner with a sheet of copper 

 and silk, three feet in length and seven inches and three fourths in 

 breadth, leaving one fourth of an inch of each end of the iron un- 

 covered for the application of the extremities of a semicircular arma- 

 ture. The barrel was sustained by a brass rod passing through it and 

 resting on a frame. The interior and exterior extremities of the 

 metallic sheet were connected respectively with the zinc and copper 

 plates of a single galvanic pair of half a square foot each. The 

 weight sustained by the magnet was eleven pounds avoirdupois. 

 After the addition of one foot and a half to the sheet, making four 

 feet and one fourth in all, it sustained fifteen pounds, or nearly forty 

 seven times its own weight. This, I believe, is several times as much 

 as has ever been sustained by a magnet of this size with so short a 

 circuit. The perfect symmetry of the transmitted current, and the 

 preservation of this property during the numerous and gradual changes 

 which may be readily made in its length, by attaching the wires suc- 

 cessively to different parts of this metallic roll, may suggest new meth- 

 ods of investigating some of the laws of electro-magnetic action. 



In the above experiment, one of the shorter edges of the rectan- 

 gular sheet of copper was soldered to the barrel for the convenience 

 of rolling it tight, and one wire connected with each of its four cor- 

 ners. In one instance the copper part of the galvanic element and of 

 the roll composed one continued sheet. In this way all wires might 

 be dispensed with. Some other laminated metals might probably be 



