1885.) NEW YORK ACADEMY OF SCIENCES. 107 



Diameter of wire = 5 millimeters of copper wire. 

 35.80 35.80 ^ .-.^ . « , • , 



62.10 + 9.^0 + 3.75 = 7^05 == ^'^^^' ^'' ^''' ^'^^^^''^^^^t. 



61 + lO.t^O + 3.80 =Wl = ^•^^^' ^'' '^'^"^ experiment. 



The labors of Marcel Deprez have both theoretically and prac- 

 tically opened the way and proved the entire feasibility of trans- 

 porting great amounts of power for long distances. Much 

 remains and will yet be accomplished in the way of cheapening 

 the first cost of apparatus required, and also of rendering it auto- 

 matic. 



Perhaps the first condition to be placed upon a motor used in 

 manufactures is that its speed shall be regular under all variations 

 of load. Now we know that with a constant field intensity H, 

 and length L, of armature wire the speed V, and the counter 

 electro-motive force e, vary together. 



e = HLV -^ = V = Constant. 



We see, then, that if wc demand a constant speed and cannot 

 vary the length of the armature wire, the intensity of the field 

 must vary with the counter electro-motive force. 



This can be accomplished by means of double enrollment, com- 

 monlv called "compound winding," patented by Marcel Deprez, 

 in 1881. 



I have already explained to you how the lines of force of the 

 field are led by iron cores surrounded by coils of wire to the spot 

 where the armature in revolving can cut them. If the whole 

 current generated in the armature is led through the coil around 

 the magnet, and then through the external circuit, the winding 

 is technically called series winding. If only a part of the current 

 is taken off at the binding posts of the machine, and led through 

 the coils around the magnet and back to the armature, the wind- 

 ing is technically called shunt winding. 



The resistance of the shunt-wound magnet coils is usually 

 much greater than the external circuit, but the number of turns 

 also is greater, and so we attain a field of equal intensity. 



Compound winding consists of the joint use of^ these two 

 methods. 



Mr. F. J. Sprague has recently (April 7th, 1885)^patented a 

 very clever combination of shunt and series windings for the pur- 

 pose of obtaining a constant speed of motion for a constant 

 potential circuit, such as is ordinarily used for incandescent 

 lighting. 



