No 119. Frederickton, N. B., Dec. 6, '05.

I would like very much to have you answer the following questions:

Is there any process for restoring the transparency to old tracing cloth that has become semi-opaque by being kept in a damp place, without interfering with the tracing in India ink already on its surface?

Is there any cheap method of photo-engraving for amateurs? Can you tell me how it is done or give me the name of some book on the subject? G. S. M.

Inquiries at several large drafting rooms fail to give any process for restoring the transparency of tracing cloth which has become opaque with age. This trouble seems to be inherent with the cloth, whether Btored in a dry or damp place, and does not affect the printing qualities, other than to make it somewhat slower.

Photo engraving requires quite an elaborate outfit as well as considerable experience and skill. The process is not one which can be taken up to good advantage by amateurs. For books upon the subject apply to Spon & Chamberlain, 125 Liberty Street, New York.

No. 120 Wheeling, W. Va., Dec. 7, '05.

What, is a time-lock, as used on large vault doors, and is one easily made?

How can I quickly magnetize some hard steel rods?

G. W. H

A time lock is a clockwork device placed on a vault or large safe door for protective purposes. Contrary to the general opinion, with a time lock it is not necessary to open a safe at a certain moment only, but the clock is set to permit the opening of the door at so many hours and minutes after the time the door is closed, and not before, and at that time the mechanism operates, but if the custodian of the vault does not wish to open the doors at that moment, he may do so at a later period. As time locks involve considerable intricate gearing, home manufacture, except on an experimental basis, is not advisable.

Wind 6 layers of 100 turns each of No. 20 cotton covered magnet wire on a mailing tube into which a rod will readily slip. The current from two or three dry or wet batteries should permanently magnetize the rods if the circuit is kept closed for 10 minutes..

No. 121. Hagarstown, Pa., Dec 12, '05.

Will you kindly answer the following questions:

Why will an induction coil not work on an alternating current if the voltage is lowered to that of direct current suitable for the coil, and the vibrator is fas tened against the end of the core so that it will not move ?

What size spark should a coil give which is 1 in. diameter, 8 in. long, the primary having two layers No. 14 wire, and the secondary 2 1/4 pounds No. 24 wire?

H.B. M.,Jr.

An induction coil properly constructed for a direct current with vibrator, should operate on an alternating current if the vibrator is secured as stated. Care should be taken to make sure that the current is not in excess of the capacity of the primary winding, otherwise it will be very likely to burn out. One difficulty in using an alternating current taken from a lighting circuit is that the frequency is so high that, unless a coil is constructed especially for use on such a circuit, the reversals of current are too rapid to permit the secondary being fully influenced by the current in the primary winding.

A coil with the dimensions and windings mentioned will give only a small spark, even if a condenser be used, owing to the large size of wire in the secondary winding. See specifications for coils in the October, '05, number.

No. 122. Minneapolis, Minn., Dec. 10, '06.

In the November, '05, number is an article on rush chair seating. I am interested in that work, but am handicapped on account of the lack of material. I endeavored to secure the rushes in this city, but no one carried them. If you can inform me where such rushes are to be obtained you will greatly oblige.

E. J. P

The rush seating of chairs is no longer a common pursuit, having been superseded by cane seats which are more quickly and cheaply made. For this reason rushes are not now obtainable of dealers, but must be prepared by the users during the season before they get brown and brittle, as the twisting into rope requires that they be strong and flexible. An article on this subject will be published if obtainable.

No. 123. Omaha, Neb., Dec. 13, '05

Referring to the article on "Induction Coils" in the Oct. '05, number, will you please give me the gauge of the primary and secondary wire in Brown & Sharpe gauge for a 1 1/2 in. spark coil. As I understand the above,mentioned article, the gauges specified therein are English gauges and vary from the American.

W. H. HThe B. & S. gauge No. 32 is nearly identical with the English gauge No. 36, but if No. 36 American gauge be used, a superior coil will result than if the equivalent gauge to the English is taken. The gauge used for the primary winding depends upon the source from which the current is taken. If dry cells are used, the primary should have three layers of No. 14 or 16 gauge. If a storage battery, two layers of No. 12 gauge will answer.

No. 124. Robinson, ILL., Jan. 21, '06.

I would like to know if ordinary cast iron would do for the field castings of an 8 light dynamo of the Edison type. S. M., Jr.

The efficiency of the field castings of a dynamo is dependent upon the metal used. A hard cast iron is the lowest, soft gray cast iron next, then annealed iron; wrought iron and steel being the best. The dimensions of the fields are determined from the grade of metal used, and specifications calculated for steel would not answer for a metal having less permeability as cast iron.

The specifications which you have should state what metal the designer intended should be used in the fields.

No. 125. Athol, Mass, Dec. 26, '05.

Will you please inform me how to fill a barometer tube so as to avoid having small air bubbles in the mercury ?

How far apart should the balls be of a spark coil rated as one inch spark ?

Where can the filings for a wireless coherer be procured, and how are they mixed? H. P.

In filling a barometer tube so as to avoid air bubbles, hold it at a low angle, put in the mercury with a small syringe such as are used to fill fountain pens, letting the mercury out slowly. As the tube becomes filled, raise the open end until at the last it is held vertical. Even then air bubbles are likely to be found in the mercury. These are removed by placing the closed end of the tube on the toe of the shoe and shaking the tube up and down rapidly but not too violently. This will slowly work the bubbles to the top.

The balls of a spark coil rated as one inch, should pass a spark at a distance of one inch, but it will probably not be a very fat spark at that distance. When using coils for sending " wireless" messages, no matter what their rated spark capacity, they are separated only about 1/8 in., a fat intense spark being what is desired.

Filings can easily be made from nickel and silver coins by filing them over a sheet of white paper with a quite coarse file, free from oil. The coins must also be cleaned before filing, and the filings must not be touched by the hands. Any dust is blown off by a bellows or fan, but not by the breath.

No. 126. Westmount, P. Q., Dec. 24, '05.

In the July, '04, number is given a diagram for wiring a call bell telephone having an induction coil. I have wired a couple of 'phones after that diagram, but they do not work properly. The signalling part works O. K., but the talking part does not, when the instruments are connected. There seems to be a "short; somewhere. May I trouble you to let me know wherein the trouble may be, outside of bad connections?

F. H. T

The trouble lies in the wiring diagram, which was correctly given in the March, '03, number. This has been mentioned several times in these columns, but was probably overlooked. A change in one wire is all that is- necessary to make the difference between success and failure.

"Liquid chalk" is a handy thing to have on the bench where there is much work to lay out on castings or sheet iron. The "liquid chalk" is a mixture of chalk, glue and water. Take a pint can and powder enough chalk to fill two-thirds of it; add clean hot water until the can is almost full, and then add about two tablespoonfuls of liquid glue and mix thoroughly while it is hot. This is much more handy than solid as it can be. put on with a brush in the same way as paint. It will not rub off in handling, and gives a nice surface to work on. The chalk must be powdered very fine or it will be rough when dry.

The use of ore of magnetic sand found on the north shore of the Gulf of St. Lawrence will be attempted, this having been made possible by a method of removing the titanium, and concentrating the sand* until it assays 80 per cent, metallic ore. However, the magnetic sand canuot be conveniently used as ore until a successful method of briquetting it has been discovered.