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PHYSICS: BARUS AND BARUS 
NOTE ON METHODS OF OBSERVING POTENTIAL DIFFERENCES 
INDUCED BY THE EARTH'S MAGNETIC FIELD IN 
AN INSULATED MOVING WIRE 
By Carl Barus and Maxwell Barus 
Department of Physics, Brown University 
Communicated January 24, 1918 
In Science, 45, 1917, p. 270, it was shown that the current induced in a 
rod about a meter long by the earth's vertical magnetic field, could be made 
surprisingly evident by the aid of an ordinary galvanometer, synchronized 
with the period of the inductor pendulum, of which the rod acts as the bob 
of a bifilar metallic suspension, including the galvanometer. Thus with a 
galvanometer showing but 10~ 6 amperes per cm. of deflection, and with a 
common period of about 4 seconds, the double amplitude of the spot of light 
reached over 20 cm. per average knot of speed of the rod. Similar results 
must be obtainable in case of a ship, where the charges accumulating at the 
ends of a long transverse rod may be tapped off into the water; or in case of 
a train where the tracks are available for dissipating charge. If, however, 
the rod is insulated, the determination of the potential difference in question 
is quite difficult. We made many attempts, as for instance the dissipation 
of charge from the ends by radium, electric incandescence, etc., the magnetic 
screening of parts of a circuit, etc., all to no practical effect. It seemed 
necessary therefore to carry the charge from end to end of the wire bodily; 
i.e., to reverse the principle of any type of induction electric machine, pos- 
sibly with the additional object of securing intensification. Here also we have 
no ultimate success to record; but the secondary results are not devoid of 
interest, the idea being to construct, with precision, a large intensifier to pick 
up whatever electrostatic potentials may momentarily or permanently exist 
in the vicinity. 
Simple Apparatus. — The apparatus (fig. 1), producing current is essentially 
cylindrical in shape, large and capable of revolving around an axle a with con- 
siderable speed. The electrical parts are of light metal, all insulated or 
screwed firmly to insulating discs attached to the axle. This frame work 
will not be shown in the figures. 
A, A', A", A'" are four insulated plates adjusted like the staves of a barrel, 
each making up about \ of its cylindrical area. Hence if the diameter of the 
barrel is D and its length L the area of each segment is A = wLD/n, if there 
are n segments. The plates A, communicate by means of metallic rods 
s, s', s", s"', with the commutator segments or posts c, c r , c" \ c'" , each sys- 
tem, A, s, c, etc., being insulated. 
When the barrel revolves, a pair of brushes b b" serve to put the two seg- 
ments A A" momentarily in contact when in the horizontal position. A 
