+ DIRECT-CURRENT DYNAMOS AND MOTORS. 
In (4) the average values of the electrical efficiency of 
motors given in Table 1 may be used: 
TABLE 1. 
EFFICIENCIES OF MOTORS. 
Z Average Average Commercial 
Size of Motor. Electrical Efticiency.| or Total Efficiency. 
+H. P. .90 75 
a r . 92 es 80 
3) sy 94 -85 
10 2 95 87 
20 ee , . 96 .88 
50 ia 97 91 
iS gt 98 92 
200 sl .98 93 
500 cp 985 94 
1000 “s .989 095 
In Table 1 the average values of the commercial or total 
efficiencies of motors of various sizes are added for com- 
parison and reference. 
It is possible to design a motor in terms of H. P., torque, 
etc., instead of K. W., current, etc., but the advantage 
of using the same formulas for both kinds of machines 
warrants the adoption of the above method. Any errors 
in assuming H’ or the efficiency are not likely to be more 
than a few per cent., and can be subsequently corrected. 
6. Forms of Armature.—The four principal types of arma- 
ture—ring, drum, pole, and disc—have already been 
described in previous lessons. At the present time, only 
the first two forms are important, the pole armature not 
being used except in small toy motors or dynamos, and 
the disc armature being adopted in but a single type of 
direct-current machine—the Desroziers dynamo made in 
France. 
The chief advantage of the ring armature lies in the fact that 
