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No. 78. Melrose Highlands, Mass., June 28, '04.

How much wire is needed for the " Sensive Relay" described on page 131 of the April, 1904, Amateur Work ? C. W. W.

About one pound of No. 36 cotton covered magnet wire is required for the "Sensitive Relay ".

No. 79. Montreal, Que., June 19, '04.

Can I put a 1 3/4 h. p. engine on an ordinary bicycle, and would it be safe ? P. H. R.

Bicycles, as they are now built, are not strong enough to sustain the engine and other equipments. To have a safe motor bicycle it would be desirable to build a complete one with heavy tubing and other parts to match.

No. 80. Franklin, N.H., June 6,'04.

I have made a chloride of silver battery as described in Vol. I., No. 12, but fail to get any current. I used chloride of silver that I purchased at a drug store and sterling silver wire. Can you tell me where the fault lies? J.L.J.

The information you send is not very definite. It is possible that the chloride of silver purchased is not a good solution, but rather more probable that in making your cell you have a short circuit ; that is, the negative and positive elements are in contact at some point. Would suggest that you look into the latter and see if the trouble does not lie there.

No. 81. Rockford, III., July 1, '04.

I take advantage of your " Correspondence" to ask a few questions which are bothering me. I think most of your electrically inclined readers will welcome more articles on the subject of wireless telegraphy, especially on its operation.

1. (a). What capacity eondenser does a one-inch coil require? (6). A two-inch coil ? (c). A three-inch coil ?

2. A two-inch spark coil requires twice the amount of secondary wire as does a one-inch coil; a three-inch coil three times as much as the one-inch coil. Does the same follow in the case of condensers?

3. If I have two 2-microfarad condensers and connect them in multiple, so to speak, is the resulting capacity four microfarads ?

4. Is a water or gas pipe a good ground for wireless telegraph work ?

5. Of what material and shape should the air conductor of wireless telegraph apparatus be?

6. Some time since an article appeared in your paper on the construction of a mercury interrupter. Since the primary coil current passed through a spring of fine copper wire, this interrupter would be useless on a coil of any size. How could it be made to interrupt a coil of twenty orthirty amperes?

7. On what size of spark coil do you consider that a mechanical interrupter is more practical than a common vibrating interrupter?

8. Does a mercury or mechanical interrupter require a condenser? R. N. M.

1. (a), (b), (c). The capacity of the condenser to be placed across the contacts of a vibrating interrupter is figured according to the amount of primary battery and speed of make and break. One must put the right capacity of condenser across the primary circuit to reduce sparking contact to a minimum and to give the best output in the secondary. The peculiarities of construction of any one cell preclude giving off hand the absolute condenser capacity necessary. Best results are obtained by experiment in the vicinity of 1 to 2 microfarad for 20 volts primary battery, for any length of secondary spark.

2. The amount of wire used in a secondary does not give spark length. It is how it is wound on in turns and layers. One cannot measure wire length in a coil giving a certain spark, and estimate that twice as much will give twice as great a spark.

3. Two 2-M. F. condensers in multiple give 1 M. F. capacity.

4. Water and gas pipes should not be used for a ground. The best ground is one buried specially for the purpose.

5. See August Amateur Work.

6. The mercury interrupter you mention was for small coils. To handle 20 amperes requires different apparatus. An article covering high current interruption will appear shortly.

7. Vibrating interrupters, if well made, will work all right on any size coil where highest frequency is of no account. Mercury and mechanical interrupters are high speed.

8. Mercury and mechanical interrupters are improved by a condenser across the make and break peints. The capacity of such a condenser is a matter of experiment and calculation, based on construction data of the coil and primary battery to be used.

No. 82, Maiden, Mass., June 1, '04.

Will you please inform me how to make the composition used in the hectograph. I know that gelatine is used, but do not know the proportions. E. P.

The composition is made from the best gelatine and glycerine. One ounce by weight of gelatine is soaked over night in cold water, and in the morning the water is poured off, leaving the swelled gelatine. Six and one-half fluid ounces of glycerine are now heated to about 200° F. (93 C.) on a water bath preferably, and the gelatine is added thereto. The heating is continued for several- honrs. This operates to expel the water and to give a clear glycerine solution of gelatine.

The composition is then poured into the tray, which must be perfectly level in order to obtain a surface nearly even with the edge. It is then covered so as to keep off the dust. The cover, of course, must not come in contact with the smooth surface. In six hours it will be ready for use.

No. 83. Boston Mass., June 14,1904.

I am making the wireless telegraph apparatus which was described in the June and July, 1902, numbers. Will you kindly answer the following question: When using the apparatus in an exhibition hall can the ground wire be in a box of damp earth instead of the ground ? I have been very successful so far in the making. N. A. T., Jr.

The ground wire must have a complete circuit to moist earth, this meaning actual ground contact. If a metal drain pipe from the eaves of the building can be reached from a window, and such drain pipe reaches down into the earth two or three feet in making connection with a sewer, you could temporarily connect your ground wire to the same, making the connection with several turns in connection with the bare metal. A painted pipe would give poor connection. Do not connect with interior pipes, like gas and water pipes, as the voltage is high and might also interfere with telephone and other low tension connections, which are frequently made to interior piping. If this is not possible, it will be advisable to have ground wire put out the nearest window, the earth end having a piece of copper plate about a foot square embedded in moist earth, the connection with the wire and copper plate being soldered.