No. 119. Springfield, Mass., Feb. 16, '06.

I have read with much interest the articles on the nine-inch reflector by Mr. M. A Ainsley, and would like to ask the following questions:

Where may the glass for the speculum be obtained?

What does he mean by the tool ? Is it a disc of glass the same size as the mirror, or is it flat when the grinding commences?

What does glass for a 6 in. and a 9 in. mirror cost?

F. S. I

Unless one desires an exceptionally fine instrument requiring imported glass, plate glass, obtainable at any large dealer in plate glass may be used. A piece 6 in. square and 1 in. thick would cost about $1.00; 9 in. square and 11/4 in. thick, about $1.50.

The tool is a piece of glass identical with that used for the lens, both being flat before grinding. As grinding proceeds the speculum takes a concave form and the tool a convex form to correspond, this being fixed by tooling the pitch as described in the articles.

No. 130. Ceylon, Minn., Feu. 12, '06.

I would like to know the size of magnet wire to use in winding the armature and field of a dynamo. Is No. 18 wire all right, or is finer wire best?

How much magnet wire does it take for an 8-light dynamo, 16 c. p. each? A. R. L.

The windings of the field and armature of a dynamo are determined by the design, materials used, and output desired. As these particulars have not been supplied by the writer we are unable to answer the inquiries.

No. 131. Batavia, N. Y. Jan. 25, '06.

With a 1/2 inch spark coil and 50 ft. aerial with proper ground connection, should I be likely to meet with any difficulty in sending signals across two city streets a distance of about 250 feet ?

How would it do to have several wires for my receiving aerial and arrange to cut out all but one while sending? L. C. H.

A half-inch spark coil, with the gap closed to about 1/8 of an inch, will transmit signals a distance of approximately 500 yards, when a very sensitive michrophonic detector is used. With a coherer made of 2-10 or 1/8J glass tube, with tight fitting brass electrodes and mixed silver and nickel filings, by proper adjusting you could operate a sensitive relay over a distance of 250 feet. Such a coherer would require more or less adjusting, but for experimental use should prove satisfactory.

It is customary in wireless stations to use one wire for sending, and a large network for receiving. There is no reason, in an experimental station such as yours, to go to this trouble unless you care to do so for the sake of making comparisons as to efficiency.

No. 132. Tisdale, Ga., Feb. 16. '06.

When I connect my aerial wire to the spark ball the spark changes from a fat white one to a thin one. Why is this? When I connect the ground on, the spark almost vanishes at times. Does this affect the sending? R. P. M.

The aerial and ground wires are radiating or dispersing all the spark discharge in the form of energy. If you use a Leyden jar bridged across the spark gap and close the gap up to 1/4 in., you will be able to send further.

No. 133. Cambridge, Mass., Jan. 29, '06.

A friend says that in magnetizing a steel rod by inserting it in a coil of wire connected to a source of electric current, that the longer the steel stays within the coil, the stronger the magnet. Another declares that magnetization is instantaneous, and that a long insertion has no value. Which is right? B. O. C

The latter is right. A greater saturation of magnetism may be obtained by tapping it with a piece of wood or a wooden mallet while being magnetized.

No. 134. Bedford, Ind., Feb, 14, '06.

Please give formula for making the red shellac coating found on finished magnet coils.

Where can I buy or lease a machine for windingffine wire into sectional coils? Do you know the price of such a machine?

Please give method of marking steel tools permanently. B.N.S.

Dissolve common red sealing wax in wood alcohol.

Winding machines, such as you desire, are not purchased outright. Coil manufacturers design and construct their own machines and do not lease them.

Coat the steel with paraffine or beeswax. Scratch the initials with a sharp point, being sure to reach the mettel. Fill the marks in the wax with a strong solution of nitric acid and water. Allow to stand for five minutes, then wash with water and remove the wax and the marks will be found permanently etched on the steel.

No. 135. S. Weymouth, Mass., Jan. 25, '06.

I would be greatly obliged if you would kindly answer the following questions:

1. Can you give me some idea as to how small a propeller I can use in a 15 ft. boat, 4 ft. beam? The engine is a small bicycle motor rated at about 1 1/2 h. p. I think most bicycle motors are regarded as high speed motors.

2. Would it be best to gear down so the propeller shaft would run slower than the engine ? If so, about what rate would be best so that the engine wouldn't race ?

3. What advantage is there in having a propeller with three blades rather than two?

4. Does the angle at which the propeller shaft passes through the boat have any effect upon the speed developed? By angle I mean that formed by the propeller shaft and the bottom of the boat. I want to use as small a propeller as possible so that the boat will draw very little water. R. A. S.

The information furnished is so meager as to make the answers only approximate and in accord with the usual practice in marine design.

1. A 12 in. propeller with three blades is about right for the size boat mentioned. The engine may or may not furnish the requisite pewer to drive it to speed.

2. Connect the shaft direct to engine; reducing gears use much power which had best be applied to driving the propeller.

3. The number of blades is determined by the area necessary to develop the required "hold" on the water. Three blades are also better balanced than two.

4. The angle of the shaft should be as small as convenience will allow, as the pitch of the blades is design-ed to work best at a low angle. In a small boat this is not as important as with large ones. It is rather doubtful if the motor you mention will drive the boat to speed especially in a strong head wind.

No. 136. Mountainville, N. Y., Jan. 17, '06.

Will you kindly answer the following questions: 1 have a small 25-volt dynamo and am trying to make a storage battery to be charged by it, but I am confronted by several troubles.

1. I have hard work making the lead oxide stay in the grids.

2. I have been unable to make a plaster mold that would stand casting over five or six grids before breaking up. The heat of the molten metal would soften the plaster next to the grid, and consequently it would drop off in thin scales when the mold was taken apart. Can I remedy this in any way ?

3. After I have the lead plates in place in the jar, would it be advisable to fill in between and around them with clean gravel to keep the oxide in place," If not, give reason. G. B.

1. If the paste is properly mixed and firmly hammered into the grids, it should remain in place, if the cells are charged and discbarged at the proper rate. Read the description in the June, 1904, number of this magazine.

2. Plaster molds are suitable for only a few castings. Make one of fine grained hard wood such as maple or birch, coating the inside with graphite powder and heating the metal only to the point where it will flow freely. Iron molds can be made at small expense, as described in the article above mentioned, and are recommended.

3. The positive plates should be fully insulated from the negative ones; even flakes of oxide which drop off are injurious, as they are very liable to cause a short circuit. The use of gravel as mentioned would have the same effect, and would be decidedly detrimental.

No. 137. Dayton, Ohio, Feb. 13, '06.

Will you please answer this question in your correspondence c< lumn :

I have a small induction coil, rated at $ in. spark.

When I first received the coil I tested It on four cells of dry battery, and it gave the required spark, but only for a minute. Since that time it refuses to give any spark at all except when the space between the secondary terminals is very small. I have tried it on five and six cells of battery, but without any better success. Can you tell me what the trouble is and how to overcome it? M. M. H.

It is evident that something has happened to the coil, whether due to your improper use or a defect in the coil cannot be determined without taking it apart and making a close examination. As the coils sent out by all reputable manufacturers are thoroughly tested before shipping, the probability is that you may have accidentally done something which has caused the trouble. First examine the connections, and especially those between the secondary winding and the discharging posts. If a careful examination fails to locate the fault a complete rewinding of the coil will probably be necessary.

No. 138. FaLL. River, Mass., March 6, '06.

In the March, 1906, issue is an article by Oscar F. Dame, in which be describes a method of building condensers by using sheets of glass 18x24 in. with tin foil placed between. Can you tell me the number of layers to get the best results for a three or four inch coil? A. F. N.

The condenser described in the article mentioned is for a particular purpose only. You do not state for what purpose you desire the condenser, but as the coil you mention is rather larger than the one for which the condenser described was used, it is presumed you have some other use in mind. Personally, we favor a condenser made of waxed paper and sheets of foil, and specifications for same are given in the October, 1905, number of this magazine in the article on Induction Coils. For a three or four inch coil a condenser having 60 sheets of tin foil 8x5 in. would answer. It would be advisable to make it up in four sections so that the whole or a part could be used as required for different kinds of work.

No. 139. San Francisco, Cal.- March 13, '06.

Can you inform me how to wind two magnets with what is called a "differential winding," which is claimed to produce absolutely no spark at break? A condenser or added resistance to same is not satisfactory as the spark still remains. I have two magnets, each wound with 400 turns of wire and use three dry eells. The object in view is to obtain the same eneigizing effect with the differential winding as when ordinarily wound, and not to use any more battery.

Is a condenser used on a spark coil to increase the spark? If so, how is it connected? Can the condenser be used to reduce the spark. T. W. H.

Differential winding is not suitable for induction coils, as one part of the winding opposes and neutralizes or balances the other part. A condenser acts as a capacity to store the energy created in the primary winding by the inductive effect of the "break" current in the secondary. In this way it has the effect of increasing the spark. If rightly proportioned to the size of the coil used with it there should be no spark at the contact points of the interrupter, provided the latter is properly designed and well made. The information you send is not sufficiently complete to tell what is the trouble, but the indications are that the fault is with the interrupter, or condenser.