A New Determination of the Ohm

Abstract

A FRESH determination of the value of the ohm in absolute measure has been made by F. Himstedt (Wiedemann's Annalen, liv. p. 305). The method employed is that which the author had used in a previous determination, and consists of passing through a galvanometer all the make or break currents induced in a secondary coil when the current in a long primary helix is interrupted a known number of times per second. A known fraction of the primary current is then passed through the same galvanometer. The primary helix in these experiments consists of a single layer of uncovered copper wire, wound, by means of a screw-cutting lathe, in a regular spiral on a glass cylinder. The turns of wire are held in their place, and the insulation improved, by being coated with shellac. As the mean of a number of determinations, the author obtains the value 106˙28 cm. as the length of the column of mercury at 0° C, having a cross section of one square millimetre, which has the resistance of 109C.G.S. units. In connection with the above-described experiments, the author has been led to measure some coefficients of self-induction, using for this purpose a modification of the Rayleigh-Maxwell method. The great difficulty in measuring a coefficient of self-induction by this method is that, in order to get a throw of sufficient magnitude to be accurately measured, it is necessary to employ a somewhat strong current. The result is that the temperature of the coil, the self-induction of which is being measured, rises rapidly, and thus the balance of the Wheatstone's bridge for steady currents is upset. Herr Himstedt gets over this difficulty by using the commutator, which he employs in his determination of the ohm, to break the battery circuit a known number of times per second, and to cut the galvanometer out of circuit while either the mike or break is taking place. In this way a steady deflection is obtained of sufficient magnitude to be readily measured, even when the current employed is between 0˙001 and 0˙002 amperes. The above method only differs from that employed by Profs. Ayrton and Perry in their secohm-meter, in that the author takes two separate readings, one with the bridge balanced for steady currents, the other when the commutator is working, instead of bringing the galvanometer deflector to zero by upsetting the steady current balance.