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CONSULTING DEPARTMENT—(Continued) As to the size of the pipe, this also varies with the system and conditions. The average number of square feet of radiation surface supplied by different sizes, is as follows: Diam. of pipe. I in. 14 in. 2 in. For steam ....40 sq. ft. 110 sq. ft. 220 sq. ft. For hot water, 30 sq. ft. 100 sq. ft. 200 sq. ft.

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Current for Duplex Telegraphy-Tele

phone Receiver Magnets Question 1: Is it not a fact that the system of duplex telegraphy, where an "artificial" resistance is used to force an equal division of current, requires about twice as much battery as an ordinary two-instrument line?

Question 2: What is the object or function of the permanent magnet of a telephone receiver?-B. E. J.

Answer 1: It is a fact that in the duplex system of telegraphy commonly used, twice as much battery is required as in a two-instrument single line of the same length. The reason for this is that the current in the artificial line, flowing as it does around the coils in a direction opposite to that in the main line, cuts down the magnetic pull of the main-line current to one-half. As an example, on the duplexes between New York and Buffalo, they use 200 volts at each end on wires having about 5,000 ohms' resistance. When opposite poles are to the line, this would give a current of 80 ma. on an ordinary line; but in the polar duplex this current is opposed in each polar relay by a current of 40 ma. in the opposite direction, so that the resultant in the relay is only 40 ma. A duplex has recently been invented in which this defect is eliminated.

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Answer 2: The permanent magnet in the telephone receiver serves to hold the diaphragm in a state of strain, so that it is thus more fully under the control of the field; also the field is intensified. The practical effect of doing away with the permanent magnet would be to reduce the distinctness of articulation. In using a permanent magnet, it exerts an attraction of its own on the diaphragm, which is increased or decreased by the current according to its direction.

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507

Complete
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Set of German Silver Drawing Instruments in One Drawing Board, 19x25 inches, seasoned pine,
Morocco Leather Pocket Case, velvet lined, with

with hardwood ledges, attached to board by bar and lock, comprising:

screws sunk in slots with metal bushings to

allow for contraction or expansion. Shellac One German Silver compass, 6 inches long,

finish, - - - - - - - - $2.00 with pen, pencil, needle point and length Ono Mahogany T-Square, 24 in. long, ebony ening bar.

lined, fixed head, shellac finish, One German Silver spacing dividers, 5 inches One 450 Transparent Triangle, 8 inch,

One 300 x 600 Transparent Triangle, 10 inch, 70 long.

One 12-inch Triangular Boxwood Scale, engine One steel spring bow dividers, 344 inches long.

divided; graduated 3-32, 3-16, 1-8, 1-4, 3-8, 3-4, One steel spring bow pen, 3%4 inches long, with

1-2, 1%, and 8 inches to the foot, and one edge needle point.

inches and 16th, with case,

One Transparent combination, Irregular or One steel spring bow pencil, 374 inches long, with

French Curve, special, needle point.

Ono Bottle Waterproof Ipk One 5-inch ruling pen, spring on upper Blade.

Ono 4-H Siberian Drawing Pencil,

One Faber's Ink Eraser, No. 1075, One case with leads.

One Faber's Pencil Eraser, No. 111,
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One dozen Thumbtacks,
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Drawing Paper, 13%x20 inches,
Twelve sheets Whitstock Drawing Paper, 11x15

inches (for practice work only),
One Erasing Shield, -

One Sandpaper Block, Total Value of Complete Outfit, $15.84

One 6-inch German Silver Protractor, 40,

Retail Price of Materials, - $8.94

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Retail Price of Instruments, $6.90

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SPECIAL PRICE TO A. S. C. STUDENTS In United States $6.95

and Canada Only) In order to secure the above special price it is necessary to be a student of the American School of Correspondence. Orders should be sent direct to the address of the manufacturer as given below, accompanied by Post Office Order or by Registered Letter for the amount and a statement that the writer is a member of the American School of Correspondence. Express charges are to be paid by the purchaser (for students in the West and Middle West express charges to CHICAGO will be prepaid.) Address:

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Builders' Architectural Drawing

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HIS work is especially designed for those who desire to learn drawing at home, a description of drawing instruments and accessories, with

rules for using them, and hints as to their care and management. Rules for laying out simple drawings and executing same. The student is taught step by step to draw to scale, first the plans, next the elevations, and finally the details of a cottage, including foundations, walls, doors, windows, stairs, and all other items required for finishing a small building complete in every particular.

A chapter and a number of plates are devoted to more elaborate work, and the student is shown by a series of easy lessons in simple language how to make more elaborate drawings. Theory is not considered in the work, nor is perspective or shading, as the author has endeavored to make the work a purely practical one for practical workmen. Nearly all the examples given are drawn to scale and may be followed as they are given or may be enlarged or reduced at the will of the student. Asan Architectural Drawing Book for real practical workingmen, who intend making draftsmen of themselves by their own efforts, this book has no equal.

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CONSULTING DEPARTMENT-(Concluded)

Step-Up Transformation Question: I have a small step-down transformer--110 volts to 10 volts-with removable 10-volt secondary. Is it possible to build a secondary to raise the voltage from 10 volts to 220 volts and 3 amperes, using the same primary? My present primary is wound with No. 20 wire of 7 ohms' resistance.-H. H. P.

Answer: In order to obtain 3 amperes at 220 volts from the secondary, it will be necessary to supply the primary with 6 amperes and 110 volts. Your primary winding of No. 20 wire would not carry this current safely, and accordingly it cannot be used. If the size of your primary winding were sufficient to carry 6 amperes safely, then there would be no reason why you could not make the changes you suggest. You would simply need to put twice as many turns on your secondary as you have on your primary. and use a size of wire which will safely carry 3 amperes.

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Milling Machine Dividing Head Question: How can I cut a triangle on a milling machine so that two of the angles shall be each 6772°, and the other 45°? In other words, how many turns must I give the index pin, if it requires 40 revolutions to one revolution of the spindle?--G. 11. 11.

Answer: Since 45° is 18th of 360°, the work must be given 18th of a turn to obtain the angle required. Since 40 turns of the index pin are required for one revolution, it would require 18th of 40 (or 5) turns to make the angle of 45°. For the angle of 6772°, we first find the fraction of a total revolution. This is :

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360 = 16 Now, 3/16 of 40 revolutions equals 712 revolutions. Therefore we must give the index pin 7 complete turns, and then 12 a turn. The half turn can easily be made by moving the pin over 12 the number

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