A New Determination of the Ratio of the Electromagnetic to the Electrostatic Unit of Electricity [Part 2] Page: 559
541-604 p. : ill. ; 26 cm.View a full description of this report.
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Rosa. ] Ratio of the Electrical Units. 559
which is equal to the percentage change in the capacity of the air
condenser, is equal to the percentage change in the resistance in the
auxiliary bridge, provided the changes are not too great. Hence,
we may proceed as follows: With the machine running at a conven-
ient speed, the auxiliary bridge is balanced as nearly as possible.
Then the observer at the auxiliary bridge changes the speed of the
motor slightly, so as to give an exact balance, and maintains the
balance until a sufficiently long chronograph run has been made.
This determines the speed which corresponds to the existing values
of the resistances in the auxiliary bridge. While this is being done
the observer at the main bridge determines the values of the resist-
ances a1 and a, in the arm a of his bridge, which will give the
nearest possible balance (I) for the condenser, leads, and commu-
tator, and (2) for leads and commutator without the condenser. Since
one-tenth of an ohm is the smallest coil in arm a it is usually
impossible to get an exact balance at this particular speed. Having
obtained the chronograph run, the resistance of a is made a1 and
the observer at the main bridge slowly alters the resistance in the
auxiliary bridge, thus causing the auxiliary observer to slowly
change the speed of the machine so as to maintain a balance in his
bridge; the main observer continues this process while continually
watching his galvanometer, until the main bridge is also balanced.
Thus a simultaneous balance is obtained, and the amount by which
the resistance in the auxiliary bridge has been altered, together
with the former chronograph run, gives us directly the speed at
which the main bridge is balanced when a has the value of a1.
Connections are now changed so as to measure the capacity of the
leads and commutator only, a is made equal to a,, and the process is
repeated. By this process a balance can be obtained in forty to
sixty seconds, while two and a half or three minutes is necessary to
obtain a chronograph record of suitable length; as a consequence
we can pass much more rapidly from the measurement of the
capacity of the condenser, commutator, and leads to the measure-
ment of the capacity of the commutator and leads only, and back
again. This reduces the chance of the commutator capacity differ-
ing appreciable in the two cases, and enables us to make in rapid
succession more determinations of the capacity in each case, both of
which are conducive to greater accuracy.
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Rosa, Edward B. & Dorsey, N. E. A New Determination of the Ratio of the Electromagnetic to the Electrostatic Unit of Electricity [Part 2], report, 1907; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc66541/m1/21/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.