70 



SCIENCE. 



[Vol. XIV. No. 339 



clamped firmly together by bolts. Little or no magnetic polarity is 

 found at the yoke, which shows that the joint is good. 



An important feature is the extremely low resistance of the mag- 

 netic circuit, which is due partly to superior quality of iron, the 

 use of which is allowed by this construction, and partly to the 

 smallness of the air-gap between the pole-pieces and the iron of 

 the armature, which is of the drum type, with teeth. In the longi- 



il0 11'/ 





tudinal recesses formed by these teeth the armature-coils are 

 wound. 



This construction increases the efficiency, allows a large reduc- 

 tion in armature speed, and improves the regulation. As showing 

 this, reference is made to the ^-horse-power machine (Fig. 3), 

 which weighs complete, with pulley, seventy pounds, and has a 

 commercial efficiency of from 80 to 85 per cent. As a shunt- 

 wound dynamo, it will generate a current of 4 amperes at no volts 

 when run at a speed of 1,800 revolutions per minute. The arma- 

 ture is wound with 7,000 inches of conductor, which is at the rate 

 of about 64 inches per volt, at the remarkably low peripheral speed 

 of 1,500 feet per minute. This showing is believed to be rarely 

 equalled in machines of the largest size. 



It may further be stated of the l-horse-power machine that the 

 drop in electro-motive force when run as a dynamo, and the varia- 



not only on the internal resistance of the armature-coils, but also 

 to an equal if not larger degree on the intensity of the field : in 

 other words, the lower the internal resistance of the armature-coils 

 and the lower the resistance of the magnetic circuit, the closer the 

 regulation. 



This is clearly demonstrated by recent experiments with a \- 

 horse-power motor on a no-volt circuit, which, with an armature 

 without teeth (the air-gap being fV of an inch, and the internal re- 

 sistance II ohms), showed a variation in speed of 15 per cent be- 



tween no load and full load ; while with an armature having teeth, 

 by which the air-gap was reduced to ^^ of an inch, but with the 

 internal resistance of armature increased to 20 ohms, it showed a 

 speed variation of only 11 per cent. The same thing is shown by 

 the performance of the i-hor.se-power dynamo cited above, and 

 also iDy details of the Prony brake test herewith. 



Prony Brake Test ^-Horse-Power Perret Motor. 



'tion in speed as a motor, are less than 5 per cent between full load 

 and no load (see details of Prony brake test). The motors are 

 usually shunt-wound, and, on constant potential circuits, run at 

 practically a constant speed, regardless of changes in load. In 

 several instances parties requiring regulation so close that they be- 

 lieved compound winding absolutely necessary, have been induced 

 to try the Perret shunt-wound machines, and have found them to 

 fully meet the requirements. 



This superior regulation is due to the fact, not always given its 

 full weight, that the regulation of a shunt-wound machine depends 



It is of course not claimed that the use of toothed drum arma- 

 tures is new ; but Mr. Perret finds that they possess some decided 

 advantages over plain armatures, in addition to those already 

 stated, as, for instance, positive driving of the coils, secured by 

 winding them in the recesses. He also finds, that, when used with 

 finely laminated field- magnets, they are free from some disadvan- 

 tages experienced in other constructions. It is quite certain that 

 such armatures, running in close proximity to solid pole-pieces, 

 would produce heating effects therein which would be wasteful and 

 very troublesome, to say the least. With laminated field-magnets, 

 all trouble of this sort is avoided. 



A strong point in favor of these machines is freedom from spark- 

 ing at the commutator, provided this is kept in reasonably good 

 condition ; and the brushes, having been once set at the non- 

 sparking point, require no changing under extreme changes in load. 

 A rocker arm for the brush-holders is therefore unnecessary, and 

 the machine is by so much the simpler. The reason for this will 

 be readily seen by electricians in the foregoing description, and lies 

 in the fact that the magnetism of the field is so powerful relatively 

 to that of the armature, that no distortion of the lines of force is 

 produced, and consequently the line of commutation remains un- 

 changed regardless of changes in load. 



A prominent electrician connected with another motor company 

 was heard to remark, after testing some of these machines, that 

 they were " harder to knock a spark out of than any he had ever 

 seen." It may be said, further, that these machines have been 



