# Page:Popular Science Monthly Volume 90.djvu/167

Popular Science Monthly

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��Then .^y.2 = .6= K maximum wind- ing space.

.6^.000049=12,245 number of turns. The diameter of an average turn of wire = <i+x=.4 + .3 = .7 or

X X

2 2

As the circumference of a circle = 3. 14 1 6 then 3.i4i6X.7 = 2.2" length of an average turn. As there are 12,245 turns then 12,245X2.2 = 26,939" the total length of wire. As the rated resistance of No. 34 copper wire is .2605 ohms per foot (see standard tables) then the total resistance equals

��26939

��X .2605 = 584 ohms.

��12

��As the resistance of any coil will vary (due to atmospheric conditions, heat radiation, etc.), it is necessary to allow a variation above and below the specified resistance. This usually amounts to from two to ten per cent.

Case II. Given spool, resistance and approximate number of turns, to find size of wire.

Assuming L = 2" d = .4. and y = i", Resistance = 250 ohms. The number of turns « = 4,000.

The simplest method is to assume a certain size wire and proceed to figure what the resistance would be, using the foregoing data. For example, using No. 30 enameled wire and proceeding as in example / the following results would be obtained:

4,000 X (.01 1 diameter) ^ = .484 wind- ing space.

.484 ^2 = .242" winding depth = x. If this winding depth should exceed .3", then wire of this size would be out of the question since the over-all diameter would exceed i" and the windings would project above the spool heads. A safe margin to allow for clearance between the top of the winding and the top of the spool head is 1/32" to 1/16".

.r-|-rf = .242"4-.4" = .642" diameter of average turn.

.642X3.1416 = 2.015" length of aver- age turn.

��4,000X2.015 = 8060" total length of

wire.

The rated resistance of No. 30 copper

wire is .103 ohms per foot. Then

8060

X. 103 = 69 ohms, which is

12 much too low.

Using smaller sizes of wires and proceeding as before, it will be found that No. 36 is the correct size of wire, although it will take approximately 4,500 turns to get the required resistance. Ordinarily a slight increase in the number of turns will be satisfactory, while a decrease is liable to cause considerable trouble. This is due to the fact that the ampere-turns will be reduced to such a point as to affect seriously the operation of the electro- magnet.

Case III. Given spool, size of wire and resistance, to find the number of turns.

Briefly the procedure would be as follows:

Estimate the maximum winding depth from the dimensions given, as in Case I.

Estimate the maximum number of turns n from the formula wa^= winding

xL space = .tL or N= . As the three

letters forming the right-hand side of the equation are known, the value of n can be easily obtained. Calculating the diameter and length of an average turn, and using the value of n found, the resistance can be obtained as in Cases I and II.

Since this value of the resistance will be above that required (if not, the given size of wire is wrong), the number of turns can be cut down to obtain the specified resistance. — F. H. Tillotson.

��A Means of Indicating the Location of a Bell Call

WHEN two push buttons operate one bell, it is difficult to judge from which button the bell is rung. If the bell is so connected that when the rear door button is pushed the bell operates on one cell, while the front door uses two cells, the difference in the volume of sound makes the location of the ring easy to distinguish.

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