Trowbridge and Hayes—Alternating Currents. 377 
ART. XLVIIL—On the production of alternating currents by 
means of a direct current dynamo-electric machine; by JOHN 
TRowsrip@e and HAMMOND VINTON HaYEs. 
Iv is often desirable to transform a direct current into an 
alternating one for the purpose of obtaining electricity of high 
tension by means of a Ruhmkorff coil, for studying the effects 
of stratifications in vacuum tubes, or for employing alternating 
currents in the study of magnetism. The’ best way is un- 
doubtedly to employ an alternating dynamo-electric machine, 
as has been done by Spottiswoode. When, however, only a 
direct current machine is available the following method can 
be employed: 
The dynamo machine, if it is not a shunt wound machine, is 
shunted by a suitable resistance. We have employed for this 
purpose thin ribbon steel about 15 cm. broad and -01 mm. in 
thickness, The remaining portion of the current from the 
machine is conducted to two brass or copper segments a a’, fig. 
This current is led to the primary coil for, for instance of a 
Ruhmkorff coil from two other segments 3, 8’. ese segments 
are fixed upon a cylindrical shaft A, fig. 1, which is stationary. 
A belt passing over the pulley B turns the wheel C upon the 
face D of which revolve four brushes which connect the adjoin- 
ing segments. The brushes a a, 0 8, are made adjustable, 
the two adjoining brushes being electrically connected, and 
4 small stream of water plays upon the segments of the commu- 
tator, The character of the spark produced by a Ruhmkorff 
coil which is marked by alternating currents has been studied 
by Spottiswoode. Without condensers in the secondary circuit 
& bright yellow glow spans the distance between the two 
terminals of the coil which partakes more of the character of a 
voltaic are than of the ordinary discharge from a Ruhmkorff coil. 
The apparatus which we used produced three thousand reversals 
&minute. This rate was too rapid for the best effects with a 
Ruhmkorff coil. It enabled us, however, to study the musical 
note produced in the cores of the electro-magnet by rapid 
reversals of the current in the electro-magnet, and also the heat- 
ing effects which have been so often studied. 
Jefferson Physical Laboratory. 
