SECONDARY CURRENTS.

35

An induction coil made by Mr. Ritchie for theStevens Institute at Hoboken, N.J., has a pri-mary coil of 195 feet of No. 6 wire. The second-ary coil is over fifty miles in length, and is madeof No. 36 wire, which is but .005 of an inch indiameter. This instrument has given a sparktwenty-one inches in length, with three largecells of a bichromate battery.

Mr. Spottiswood of London has just had com-pleted for him the largest induction coil evermade. It has two primary coils, one containingsixty-seven pounds of wire, and the other eighty-four pounds, the wire being .096 inch in diameter.The secondary coil is two hundred and eightymiles long, and has 381,850 turns. This coil ismade in three parts, the diameter of the wire inthe first part being .0095 inch; of the secondpart, .015; and the third part, .011. With fiveGrove cells this induction coil has given a sparkforty-two inches long, and has perforated glassthree inches thick.

The electricity thus developed in secondarycoils is of the same character as that developedby friction ; and all of the experiments usually