As there is still some discrepancy in the values of the ohm obtained by able workers using various methods, it seems desirable to put forward any criticisms that may suggest themselves, in the hope that the causes of disturbance may thus come to be better understood. I propose accordingly to make a few remarks upon the paper of Professor Himstedt, translated in your November number, not at all implying that his results may not be as good as any other, but rather in order to raise discussion on certain points which the author may be able to treat satisfactorily when he publishes a more detailed account of his work.

The leading feature in the method of Prof. Himstedt is the use of a commutator, or separator, by which the make-and break-induced currents are dissociated, one or the other passing in a stream at equal small intervals of time through a galvanometer, by whose aid their magnitude is appreciated. The instrument works with mercury contacts. When I first considered the methods available for the solution of this problem at Cambridge in 1880, I found ready to my hand an ingenious apparatus, contrived by Prof. Chrystal for this very purpose. The contacts were effected by metallic dippers, controlled by eccentrics, and passing in and out of mercury cups.