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An overview of the instability of laminar Newtonian liquid jets in air is given, and the experimental procedures used by previous investigators are described. It is shown that empirical data are still needed for design purposes. For a systematic improvement of existing mathematical models through experimental data, a facility is designed, constructed, and tested for measuring the breakup length of jets and the size distribution of the drops. It consists of a flow system, an electro-optical detector and a signal conditioner. The performance of the electro-optical detector is shown to be good. The output signal from the electronics is a pulse train that corresponds to the train of drops passing through the detector. Anomalous "doublet" and "triplet" pulses in the pulse train are studied. For small nozzle, the anomalous pulses are attributed to large drops and/or small satellite drops passing through the detector simultaneously. For large nozzle, the anomalous pulses may come either from two partially overlapping drops or from a single drop with one or more waists.