Measurement of very short time intervals; a partial deflection method.---After briefly reviewing the usual method of determining time intervals of the order of microseconds, the author describes a method in which the potential difference of a condenser after it has been discharged for the interval of time to be measured, is accurately determined by connecting the condenser through a ballistic galvanometer to a known potential difference, which is made nearly the same as that to be measured, so that the small throw observed merely measures the small correction to be applied to the known potential difference to give the unknown. A steady current through a known resistance is used as the source of known potential difference. The arrangement of circuits is described and also a gang switch which simplifies the experimental manipulation and eliminates possible error from absorbed charge in the condenser. The working equation for computing the time interval from the observations shows that the attainable accuracy depends on the accuracy of a resistance, a resistance ratio and a capacity. The method was tested with the aid of a Helmholtz pendulum and it was shown that an interval of 250 microseconds may be measured with such accuracy that the probable error of each observation need not exceed 0.15 per cent. The precision was such as to enable the determination of an overall temperature coefficient of the Helmholtz pendulum and accessories amounting to about 1 microsecond per degree for the interval just mentioned.Measurement of the rate of detonation of explosives; a source of error.---An attempt to apply the above method to measure the rate of detonation of dynamite and trinitrotoluol gave inconsistent results, probably because, on account of the intense ionization, the condenser continued to discharge through the gas after the wire was broken.