as may be seen from Fig. 17, consists of two flanged iron tubes AA whose end consists of a convex segment of a sphere accurately turned to recieve the armature. Coils of wire CC which are in the outside circuit and through which the entire current flows are wound upon the tubes. After the armature is placed between them the two tubes are bolted together by heavy wrought iron bars BB and the whole carried on the frame work PN shown also at PN Fig. 23. Now a little magnetism only remains in the wrought iron bars and iron frame works when the armature first revolves, but the current even though slight, going through the coils makes an electromagnet out of each tube and heavily magnetizes the wrought iron bars and in two or three seconds after the armature first rotates it is entirely surrounded by a heavy magnetic field. One of the good points of these field magnets is that but very little magnetism is lost as compared with most other dynamos and since it takes power to maintain a heavy magnetic field, this dynamo is in this respect very economical.