No. 148. Marbass, N. D., April 29, 1906.

Will you please tell me what size of wire to use and the amount in the primary and secondary windings for an induction coil to give a one-half inch spark? Would a condenser be necessary? If so, how is one made? Where can the tinfoil for Leyden jars be obtained? E. D. C.

Specifications for many sizes of coils, including one half-inch spark, are given in the Oct., 1905, number of this magazine. For secondary windings, No. 36 B. & S. gauge single cotton covered wire may be used if extra care is taken in applying insulating wax. A condenser is necessary. Directions for making are given in June. 1906, number. See our premium offer for tin foil.

No. 149. San Diego, Cal., April. 29, 1906.

Will you please answer the following questions through the correspondence columns: Could the amateur runabout described in the March, 1905, number Amateur Work, be made by a boy 12 years of age, if the iron work was bought ready made and finished? Where could these parts be obtained? Is there any place where I can obtain the parts of runabouts ready to put together? H. S.

Age has less to do with.making a runabout, or any other equally complicated machine, than has mechanical skill and experience. Our experience in this matter, and considerable time has been spent in investigating the subject, leads us to the conclusion that it is much more satisfactory to watch the advertising columns of the Sunday papers published in the larger cities, and buy a second-hand car, when a good trade offers, rather than attempt to make one. The agencies of standard makes of machines frequently have to take small cars, and such second-hand cars are sold at low prices. The difficulty, even in the largest cities, of obtaining certain parts, makes the expense of constructing a car great enough to nearly offset the saving in making one. Damaged cars can frequently be bought at very low prices, and a good mechanic can make the repairs necessary at a small expense. As conditions now are, the making of a car cannot be recommended.

No. 150. Buffalo, N. Y.. April 29, 1906.

I have a dynamo that gives 35-40 volts and 3 amperes, The armature is wound with No. 20 single covered magnet wire. I wish to increase the amperes and lower the voltage about 10 volts, but without changing the wiring on the armature, if 1 can. How many storage batteries can I charge with a current at 20 volts J. P

It will be necessary to rewind the armature to obtain an equivalent output to that at present, at the same speed. Use No. 18 gauge wire, and at a slight increase in speed you will get about 25 volts and 5 amperes. You can charge 8 cells of a storage battery with a cuvrent at 20 volts.

Ng. 151. Newton, ILL., May 18, 1606.

I have a 1-12 h. p. motor on my peanut roaster, and the current for running the motor is supplied by a battery of 12 dry cells. Will a spark coil strengthen the battery, or what will give me a greater current ? Please tell me the use of a condenser. G. W. H.

A spark coil transforms the current from a low voltage and large amperage to high voltage and low amperage, with slight losses in the operation. Introducing a spark coil into the battery circuit would be a positive detriment, as the current from the coil would not be suitable' for running the motor, You need a different type of battery; one giving a large, steady current on closed circuit, as the motor is probably in use so much of the time that the dry batteries polarize more or less, and consequently do not properly run the motor.

If the expense and weight are not objectionable, accumulators would be the most suitable, and they could be recharged at the lighting station at such times in the day as they are not in use. If facilities for re-charging are not convenient, get Edison-Leland, or some similar type of battery, or make a bichromate battery, as described in the June, 1904, number of this magazine. The objection to the latter form of battery is that the zincs must is be removed when the battery is not in use, as action continues whether a current is being taken off or not, so long as the zincs remain in the solution. The function of condensers is fully described in the May, 1906, number. No. 152. Buffalo, N. Y., May 14,1906.

Will you kindly answer the following questions regarding the " Telegraph Recorder" described in the Sept., 1904, copy of Amateur Work; Would the works of an ordinary alarm clock be strong enough to move the tape? Where can I purchase a small pen of the kind used on paper ruling machines? Will you kindly give directions for making the trip to be used on the clock gear? Kindly mention the name of some firm from whom I may purchase a length of Wollaston wire as used in electrolytic receivers. W. H. C.

The works of an ordinary alarm clock, if in good condition and not of too small a size, will move the tape without trouble. The ruling pen can be purchased of any ruling firm in your city. Ask any stationer where ruling is done, and apply at the address given. An article describing a trip for the clock will be published as soon as prepared. We know of no firm where Wollaston wire can be purchased at retail. As an accommodation we will send you three feet of .002 gauge wire from our own supply for 25 cents. No. 153. Providence, R. I., May 14, 1906.

Will you please inform me of a quick drying varnish which can be used to protect steel during the process of etching names. H. DAsphaltum varnish is the kind generally used for the purpose named. Paraffine wax can also be used, the latter being more easily worked when marking out the letters.