There are many methods by which a resistance can be measured absolutely in the electromagnetic system of units, and all of these necessarily involve absolute measurements of a length and of a time. The length may be the axial length or radius of a coil, or the radius of a disc, or it may involve all of these, and the time may be the time of vibration of a magnet, or of a rotation of a coil, or of the period of an alternating current. In any case, the precision secured in the measurement of a resistance depends primarily on the accuracy obtained in these measurements of length and of time. The first absolute measurements of a resistance were made by Kirchhoff in 1849, but it is to W. Weber that we owe the first distinct proposal (in 1851) of a definite system of electrical measurements according to which resistance can be measured in terms of an absolute velocity.