Practical Physics Volume 2; Elementary and advanced measurements in electricity and magnetism

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...to e' and g' or to c and g, r is a regulating rheostat by means of which the battery M may produce any desired electromotive force between the points f and h, and V is an accurate direct-current voltmeter or potentiometer for measuring the voltage across fh. To make a measurement, throw the switches SS so as to connect both comparator wires to the terminals of R (the desired positions of the switches SS are shown by the full lines in the figure), and then with the current that is to be measured flowing through R, adjust the rheostat /f until the comparator mirror The exact method of using the A.C.-D.C. comparator departs from this statement slightly, as will be seen later. f Caution! Start with maximum resistance in I and r, and thus avoid burning out of comparator wires. shows no deflection. Now throw the switches so as to connect the comparator wire cd to the direct-current source (the correct positions of the switches are shown by the dotted lines), and regulate the rheostat r until there is again no deflection. Then the electromotive force E read by the instrument V is equal to the effective electromotive force across the terminals of the standard resistance. The resistances of the leads e'/' and g'h' must be the same as the resistances of the leads ef and gh. The effective value of the current flowing through the ammeter A is then equal to (E/R) plus the current which flows through the wire ab. The current which flows through the wire ab varies with the electromotive force E and it may be repreresented as a function of E, namely /(-£). The current through the ammeter therefore is equal to (E/ R) + /(E). The value of /(E) is equal to E divided by the resistance of the wire ab, but this resistance varies slightly with the voltage on account of the c...