An improvement in the Siemens armature was made four years later by Von Hefner-Alteneck, an engineer in the employ of Siemens. This improvement consisted in winding on the iron core a number of coils similar to the one coil of the Siemens armature, but wound in different directions. This is called the "drum armature" (Fig. 48). The heating of the core is prevented by building it up of a number of thin iron plates insulated from one another and by air-spaces within the core. The insulation prevents the small currents from flowing around in the core. The air-spaces serve for cooling. The drum armature was a great improvement over both the Siemens and the Gramme armatures. With the Siemens one-coil armature there is a point in each revolution at which there is no current. The current, therefore, varies during each revolution of the armature from zero to full strength. In the Gramme armature only half the wire, the part on the outside of the ring, receives the full effect of the magnetic field. The inner half is practically useless, except to carry the current which is generated in the outer half. Both these difficulties are avoided in the drum armature. The dynamos of to-day are modifications of the two kinds invented by Siemens and Gramme. Many special forms have been designed for special kinds of work. [Illustration: FIG. 48--A DRUM ARMATURE, SHOWING HOW AN ARMATURE OF FOUR COILS IS WOUND] Edison's Compound-Wound Dynamo Edison, in his work on the electric light and the electric railway, made some important improvements in the dynamo. The armature of a dynamo is usually turned by a steam-engine. Edison found that much power was wasted in the use of belts to connect the engine and the dynamo. He therefore connected the engine direct to the dynamo, placing the armature of the dynamo on the shaft of the engine. He also used more powerful field-magnets than had been used before. His greatest improvement, however, was in making the dynamo self-regulating, so that the dynamo will send out the strength of current that is needed. Such a dynamo will send out more current when more lights are turned on. Whether it supplies current for one light or a thousand, it sends out just the current that is needed--no more, no less. It will do this if no human being is near. An attendant is needed only to keep the machinery well oiled and see that each part is in working order. Edison brought about this improvement by his improved method of winding. This method is known as "compound winding."