Popular Science Monthly 



151 



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. 



