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Reference is made to some of the more important contributions that have been made to the s ubj ect of precise measurements of electrical resistance. The sensitivity of bridges when used with the modern high-sensitivity moving-coil galvanometer is discussed rather fully . Special consideration is given to t he methods and apparatus used and to the procedures followed in the National Bureau of Standards in those comparisons in which the precision desired is of the order of 1 part in a m illion.The more important factors limiting the precision of the comparisons, such as load coefficients, terminals, and contacts, thermoelectromotive forces, insulation, and the optical system of the galvanometer, a re disc ussed rather fully. A method of analysis of networks containing both li near and nonlinear four-terminal conductors is given, and the theoretical basis for the experimental procedure used in detert:"\ining the effect of slight defects in the insulation is pointed out. This is followed by a bri ef d:scussion of Ohm's law from the s tandpoint of precise resistance measurements and by a brief disc ussion of units of resistance. Finally, reference is made to more than 100 publications having a more or less direct bearing on the subj ect of resistance comparisons. CONTE NTSPa&,e

A procedure which may be followed in the development of a general design for a seismometer, to give approximately a specified performance, is illustrated by a concrete example. In this design the steady mass is about 500 g; the arrangement is such that the period may be determined and the damping adjusted from the recording station, which may be at a distance from the seismometer; and the magnification and its variation with the period of earth displacements, in the range from 2.5 to 60 seconds, is substantially the same as is given by a seismometer of the ordinary type having a magnification for shortperiod displacements of 1,250, a period of 12.5 seconds, and critical damping.Preliminary results obtained with an experimental seismometer constructed substantially in accordance with this design are given, and a photograph of the instrument is shown.

A vacuum-tube amplifier of tbe type described is a simple and inexpensive means for compensating small alternating voltages. Two such amplifiers and an iron-cored inductor witb air gaps are used as a choke in a circuit baving a resistance of approximately 20 ohms and in wbich there is a direct current of approximately 1 ampere. Tbe effectiveness of the arrangement is such that rapid variations of the potential drop amounting to 0.1 volt in a part of the circuit result in variations in the current of less than 1 microampere.One of the methods now being used in this Bureau in an absolute measurement of electrical r esistance requires that thei test current through a standard resistor be kept very nearly constant over successive time intervals of a few hundredths of a second. This requirement must be met even though, in the same circuit, there are rapid variations of resistances and voltages of sufficient magnitude to disturb the constancy of the current unless their effects are nulli-fied. The purpose of this paper is to describe briefly FIGURE l. Simplified circuit containing a resistor, th I d t h ld R, in which it is necessary to maintain a direct e means emp oye 0 0 current which, over short intervals of time, is conthe current constant. stant to one part in a million while there is aThe problem may be visu-pUlsating potential difference between p and, q. alized by referring to figure 1. In this figure, Erepresents a battery supplying direct current of approximately 1 ampere through the l-ohm resistor, R. For the purpose of measurement, it is necessary that the fluctuations of current in R should be less than 1 microampere from its average value over successive time intervals of a few hundredths of a second. However, the operation of a rotating commutator and other parts connected to p and q, but not shown in figure 1, constitutes in effect a source of alternating voltage, V, between the points p and q. Although this alternating voltage has a fundamental frequency of approximately 45 cycles per second, 41