558 



Progress of Science. 



[Oaober, 



AMERICAN CORRESPONDENCE. 

 By Professor Leeds, Stevens Institute of Technology. 



Large Induction-Coil. — I have obtained from Dr. Wahl, the Secretary of the 

 Franklin Institute, the following details of a very large induction-coil, probably 

 the most powerful now existing, which has lately been constructed for Professor 

 Morton by Mr. E. S. Ritchie, of Boston. Mr. Ritchie, as is well known, was 

 the first, by several fundamental improvements, to make the induction-coil an 

 efficient and reliable source of electricity ; and it would seem that by con- 

 stant attention to the subject, he has succeeded in making the most perfect 

 instrument yet constructed. The coil now described, containing but 44^ miles 

 of wire, 40 inches in length, and weighing about 206 pounds, gives, with but 

 three cells, sparks 21 inches in length, and, after several months of constant 

 use and severe tests, is in perfect condition. 



The accompanying woodcut, which is a faithful copy from a photograph of 

 the coil, with some familiar objects as standards of comparison, will give a 

 general idea of its structure and arrangement. It is made in three parts, one 



Fig. 21. 



consisting of the condenser, enclosed in a mahogany case, as shown in the 

 foreground carrying on its upper surface the automatic and hand-break piece, 

 commutator, &c, and two others, forming the coil itself. These last are so 

 arranged that they may be separated from each other, and used apart or united 

 for quantity. The pole cups, by which the halves of the coil and condenser 

 are united, have been omitted by the engraver on account of their confusing 

 effect, as they were superposed by the perspective of the picture. 



The particulars of construction are briefly as follows : — The iron core con- 

 sists of iron wires, about T x F th inch thick, and weighs 14 pounds. The thickness 

 of the wires is immaterial, except as it affects their annealing. These wires 

 are not insulated from each other, and are simply bound together with a 

 covering of oil-silk and cloth for strength. 



The primary wire is 200 feet in length, and 0*1655 of an inch or about £th 

 of an inch in diameter, and weighs 17 pounds. The secondary wire is 234,100 

 feet, or about 44^ miles, and 0-07 inch in diameter, and weighs 44^ pounds. 

 It was made of Lake Superior copper, of the best electrical conductivity, and 

 is covered with white silk. It is wound according to the plan devised by 

 Professor Ritchie, in a series of spirals representing the thickness of the wire 



