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BELL SYSTEM TECHNICAL JOURNAL 



of spherical shells; this part was thereby made very rigid so that it 

 should move as a unit up to high frequencies. 



Construction of the Driving Coil 



For the driving element of loud speakers either a moving coil or a 

 moving armature is commonly used. The latter is in general satis- 

 factory if driven at a small amplitude. However, where large powers 

 are involved, the moving coil drive can be much more simply con- 

 structed so that it is free from amplitude distortion ; it has the further 

 advantage of having a resistance nearly constant with frequency and a 

 practically negligible reactance. These were the primary reasons 

 for our choosing this type of drive. 



The coil that was used in the re- 

 ceiver consisted of a single layer of 

 aluminum ribbon 0.015 inch wide and 

 0.002 inch thick wound on edge as 

 shown in Fig. 4. The turns were 

 held together with a film of insulating 

 lacquer about 0.0002 inch thick, thor- 

 oughly baked after the winding was 

 completed. This type of coil has the 

 following advantages. It is self-sup- 

 porting, no spool being required; 90 

 per cent of the volume of the coil is 

 occupied by metal; the distributed 

 capacity between turns is small, giv- 

 ing a coil whose impedance varies 

 only slightly with frequency ; the 

 metal is continuous between the cylin- 

 drical surfaces, allowing heat to be conducted rapidly outward from 

 the center of the winding and diminishing the possibility of any 

 warping of the coil; it can be accurately made to dimensions, thus 

 permitting small clearances between the coil and the pole pieces. 

 Small clearances not only permit the use of a comparatively small 

 magnet but they facilitate the dissipation of heat. This latter effect 

 is shown in the curves of Fig. 5. These curves give the temperature of 

 the coil as a function of the power input for the coil in open air {A), 

 and when it is placed between annular pole pieces with clearances of 

 0.010 inch between the cylindrical surfaces {B). 



Fig. 4 — Receiver driving coil. 



