688 REPORT—1904. 
ing the value and phase of its rotor current are perfectly well known. (See 
‘Electrical Engineer,’ August 12, 1904.) 
If the motor is arranged so that its stator may be excited with a continuous 
current equal to 1:41 A ccs 2d amperes (where A is the normal magnetising 
current of the motor, d the angle between the brush line and the polar axis), and 
the rotor is arranged to be driven by a continuous-current motor, then the follow- 
ing tests may be performed :— 
I. Calibrate the C.C. motor, so that the power required to drive it at any 
particular speed is known. 
II. Connect it to the A.C. motor shaft (the A.C. motor not being excited), 
and measure the power required at various speeds (friction and windage), 
III. Drive as above (in II.), but with the rotor on open circuit and stator 
excited with continuous current, as explained above. (Rotor hysteresis and eddy 
losses, hence also stator hysteresis and eddy loss.) 
IV. Close the rotor circuit by connections which revolve with the rotor, and 
excite (as in ITI.), and run at what would be synchronous speed. This gives the 
rotor current at standstill (if desired, for various values of d). It also gives, if 
the speed is varied, the rotor power factor at standstill—a most important point. 
V. With the same continuous current the resistance of rotor and stator may 
be measured. 
Thus all the losses of the motor are completely known, as well as the rotor 
current and power factor, and the performance of the motor may be predicted 
exactly, 
The method enables a very satisfactory test to be made, when alternating 
current of the right voltage and frequency is not available, which is an important 
point, especially in England, where the frequencies are so varied as to make it 
necessary to have a great deal of apparatus to test even a moderate run of motors. 
No wattmeter is necessary, which is another advantage, and in technical colleges 
it will be found that students will grasp the working of alternate-current apparatus 
in general, if such methods are used to bring home the close connection which 
exists between it and similar direct-current machinery. 
A model of the apparatus was shown. 
6. The Action of Lightning Strokes on Buildings. 
Sy Kituineworta Hepces. 
‘ Published in the Electrical Engineer, September 2, 1904. 
