which it takes 100 H. P. to drive the alternator and exciter. The exciting current of 22 amperes and 135 volts is supplied by a small Brush are light dynamo. The regulation is effected by sending part of the current from the exciting dynamo through a shunt box of varying resistance operated by hand. Very little regulation, however, is required, as the machine is nearly self-regulating from no load to maximum load. The armature (Fig. 10) is stationary and the fields revolve, this being the only A. C. machine in America so constructed. Such a construction has many advantages: in the first place the revolving part has only a low tension current passing through its coils, and being heavy and perfectly balanced acts as a fly wheel and regulator when from any cause there is a tendency in the prime motor to slacken speed. The wires from the switchboards are permanently attached to the ends of two coils of the armature. The armature coils are light and independently bolted to the ring which holds them in position between the field bobbins. Should an accident happen to one of the armature sections, the latter may be replaced in five minutes by one man, the weight being only a little over 20 lbs. The armature core is composed of fibre in segments on which are wound copper ribbons insulated by silk and shellac. The total weight of the armature ribbon is 63 lbs., and of the complete machine 5120 lbs.

There are 12 cores of alternating polarity on each side of the field wound with 618 lbs. of copper wire. These cores are of wroughtiron bolted to two iron plates, and the weight of the twelve is 456 lbs. The output of the machine, 60,000 watts, is at the rate of 11.71 watts per lb. gross weight: 88.1 watts per lb. of the total copper on armature and cores, and over 952 watts per lb. of the copper ribbon on the armature.

The Royal Electric Co.'s Dynamo (Fig. 11).

Sometimes the machines of the Royal Electric Co. for A. C. work are made with the exciter on the same shaft as shown in the cut, but more frequently the exciter is separate. In the 1200-light dynamo with separate exciter, the active iron in the fields weighs about 3400 lbs., and the iron rings of the armature (which is a hollow cylinder built up of laminated wrought iron) weigh 1250 lbs. The radial depth of these rings is 5". When turned up ready for winding the armature is 24ğ" diameter and 18" long. The diameter when wound is 251" over the bands, and, as the internal diameter of the field bobbins is 251", there is a clearance of full" all round. The fields are wound with 845 lbs. of No. 8 B. & S. gauge wire; the armature winding con

sists of 32 lbs. of No. 11 B. & S. gauge wire. The speed of the machine is 1200 revolutions per minute, and the exciting current at full load is 24 amperes and 90 volts-say 2160 watts or nearly 3 E. H. P. The standard output of the machine is 70 amperes and 1020 volts, or 7.5 watts per lb. gross weight: 81.41 watts per lb. of the total copper wire and 2231 watts per lb. of the wire on the armature.

RATE OF ALTERNATION OR PERIODICITY.

In the Brush dynamo the periodicity is 110, while in the Fort Wayne, Westinghouse and Thomson-Houston machines the periodicity is from 125 to 136.

The European practice as to the rate shows considerable variation Messrs. Ganz & Co., of Buda Pesth, in the Zipernowsky system use 42 cycles; Mr. Ferranti uses 68; Lowrie-Parker work their machines at 80; and Mr. Mordey runs his at 100.

CONVERTERS.

Westinghouse Converter (Figs. 12 and 13).

The Westinghouse converter is of the shell type. In the 40-light converter, the core is built up of plates .017 inch thick, of which the total weight is 81 lbs. The primary winding of No. 16 wire weighs 10 lbs., and the secondary wire is No. 3 gauge, 13 lbs. in weight. At 50 volts and 40 amperes this converter gives an output of 19.09 watts per lb. of active material. The P. D. required between primary terminals is 1000 volts, developing an E. M. F. of 50 volts at the terminals of the secondary coil.

Recent tests by Dr. Louis Duncan, of John Hopkins University, show 95 per cent. efficiency in a 40-light converter at full load, and only 84 watts loss with no load, and 90, 87.6, 83.3, and 70.7 per cent. efficiency in a 20 light converter at full, three-quarter, half and quarter loads respectively.

Brush Converter (Figs. 14 and 15).

The Brush converters are of the core pattern, with the exception of the two of smallest size. The iron wire in the core of the 50 light converter weighs 92 lbs., the primary wire weighs 20 lbs., and the secondary winding 11 lbs. At 100 volts and 30 amperes its output is thus 24.39 watts per lb. of active material. The case weighs 100 lbs. The 75-light converter core of iron wire weighs 115 lbs., primary winding of copper wire 21 lbs., and secondary winding 15 lbs. At 100 volts and 45 am