(No. 6.) Elkhart, Ind., Jan 27, 1902.

I would like to know whether it is possible to make a call-bell telephone out of the following materials suitable for a line one mile long:

Two ordinary electric bells,

Two telephone receivers,

Two push-buttons, and wire and batteries sufficient for one mile. Also give a diagram showing the method of connecting the wires. C. H. W.

The materials you mention are suitable only for a short line. If you want to run a line for a distance of one mile, you would need 10 or 12 cells to your battery to get sufficient energy. These would cost more than would a cheap magneto-telephone. You would also need double-contact push-buttons. A series of articles on telephony are in preparation which will give complete information about the equipment of short lines.

(No. 7.) . Mendota, III., Feb. 5, 1902.

I wish to make an induction coil which will give a spark about one-fourth of an inch long. What sizes of wire are best for each coil, and how much of each is necessary? Is it necessary to have a condenser? Will such a coil operate the apparatus for wireless teleg-raphy described in the November number of Amateur Work? M. E. F.

The following are the dimensions for a coil giving a one-half-inch spark, this size being given as it requires but little more work or material than the smaller size, and is much more satisfactory in operation.

Length of coil, 51/2 inches

Length and diameter of core, 6 x 5/8 inches

Condenser, 40 sheets tinfoil, 6x3 inches

Primary coil, 2 layers cotton covered, No. 18 wire Secondary coil, 1 pound, silk covered, No. 40 wire

A condenser is desirable when a large, fat spark is desired. It would not be advisable to make the apparatus described in the November issue, other than to illustrate the apparatus used by Dr. Hertz, which was the idea in view in that article. The description of how to make a wireless telegraphy apparatus that will operate over water for a distance of about two miles is now in preparation.

(No. 8.)

Can you tell me the process used for finishing ma-hogany to obtain the very dark, rich crimson and black colors seen on the best of mahogany furniture, etc. ? Also, can birch be finished in the same manner to imi-tate mahogany ? F. W. P.

To finish mahogany, maple, birch, white wood and other woods so as to obtain the dark, rich mahogany colors mentioned by this correspondent, proceed as follows: Make a solution of bichromate of potash, using about two ounces of potash to a pint of water. Apply this to the wood before staining. If a very dark tint is desired, several coats may be necessary. It would be well to take a small piece of the wood used and experiment to determine the necessary coatings. The solution may be made either stronger or weaker; the stronger the solution, the darker the tint. After the coating of potash is dry, smooth the wood with fine sandpaper, then apply the stain, which may be red or brown as desired, and finish with varnish or polish. A weak solution of nitric acid is sometimes used in place of the potash.

Marconi claims to have perfected his telegraphy apparatus so as to avoid interruptions by other systems.