No. 96. East Orange, N. J., June, 2, 1905.

I have a small 1 in. induction coil which I recently purchased, and wish to know if same is good for wireless communication over water for two miles? If not, how far?

What do coherers sell for generally at the stores?

J. F. S

A good 2 in. spark is sufficient for the distance you mention, provided you have a sensitive receiver and aerial wire at least 25-feet high; also a good ground connection at each station.

Coherers are catalogued generally at from $3.50 to $6.00, according to sensitivity and reliability. Address any of our electrical advertisers for price lists.

No. 97. Fort Collins, Col., June 9, 1905.

Are dry batteries better than wet batteries for small spark coils?

What insulating material is recommended for saturating a primary core and winding for a 6 in. coil?

What voltage is supposed to be given by the discharge of a 2-in. induction coil ? G. R. N.

Dry batteries, because of portability, commend themselves for spark coil operation in wireless telegraphy. Fuller or Edison batteries give excellent results.

An excellent and cheap way to insulate a primary is to wrap one layer of thin cloth between each layer of wire on the primary. After winding is completed, wrap 3 layers of cotton cloth over the winding and tie tightly with thread or twine. Then heat to boiling 1/4 lb. of paraffine wax and two ounces of resin, and immerse the core and winding for twenty minutes. Then fashion from pasteboard a tube like a mailing tube, 1/4 in. larger in diameter than the primary and 1 in. longer. Insert the primary in the tube, stand on end and pour the following mixture into the tube, permitting same to thoroughly harden before removing the pasteboard form. For the mixture, add 2 ounces of resin to the remaining portion of the paraffine mixture previously mentioned, and stir thoroughly. The addition of an ounce of hard South American wax is recorr-mended by many coil makers.

A 2-in. spark is commonly called 25,000 volts. Until recently there has been a wide difference of opinion as to the voltage for any given length, owing to lack of instruments for measuring high potentials. Some of the text books of ten years ago speak of 50,000 volts to the inch, but recent investigations show these measurements to be excessive. Even today, experts disagree as to the best way of calculating the pressures at spark gaps of various lengths. The figures we give seem to be reasonably accurate.

The coal production of the United States is now about 1,000,000 tons per day, and the railroads consume about 40 per cent of it.