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presents p r o b l e m s s e l d o m e n c o u n t e r e d i n ordinary l i / 2 χ 4 0 microsecond i m p u l s e w a v e testing. T h e prob l e m consists of o b t a i n i n g linearly rising steep-front voltage oscillograms w h i c h faithfully r e p r o d u c e d , to a k n o w n scale, the voltage w a v e as seen by the test speci m e n . Stray or residual divider parameters, as w e l l as the m e t h o d s used in g r o u n d i n g , shielding, s u p p l y i n g power, and synchronizing the oscilloscope, m a y i n t r o d u c e spu rious responses i n t o the voltage oscillograms.

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impulse waves, when the time to flashover is less than 1 microsecond (/xsec), presents problems seldom encountered in ordinary testing where flashover generally occurs on the tail of a conventional IV2X 40 Atsec wave.In the fractional microsecond range, the voltage applied to the test specimen is generally increasing at a rapid rate at the instant flashover occurs. This means that the current through a resistor potential divider is likewise increasing at a corresponding rate, and hence the residual inductance in each element of this divider will generate an L dl/dt voltage which is superimposed upon the IR voltage of the element. If the high and low sides of the resistor potential divider have different L/R ratios, the voltage division at the instant of flashover may be somewhat different than would be indicated by the resistance ratio alone. Thus, flashover voltages measured with such a dividei may be somewhat in error, if only the resistance ratio is used in calculating the crest voltage.Errors due to residual inductance in the divider elements become negligible when flashover occurs on the tail of the wave. As the applied voltage pulse passes through its crest value prior to flashover, the L dl/dt component of voltage becomes very small, and hence, at crest voltage, the divider ratio is nearly equal to its resistance ratio. Thus, these crest voltages are accurately indicated.In addition to amplitude errors caused by residual inductance in the potential divider, the presence of "hash" on the front of the oscillograms becomes increasingly annoying as the steepness of the wave increases, and the time to flashover becomes smaller and smaller. These extraneous signals obscure the start of the voltage wave and generally make measurements of time to flashover inaccurate and difficult. These same disturbances have little effect, when flashover occurs on the tail of the wave since they are generally gone long before crest voltage is reached.The primary purpose of the doubly shielded isolated screen room which now houses the cathode-ray oscillograph (CRO) in the High Voltage Laboratory of The Ohio Brass Company at Barberton, Ohio, is to eliminate these extraneous signals. The Isolated Screen Room InstallationThe ideal place to locate the transient deflection plates of the CRO would be right across the low side of the potential divider with leads approaching zero in length. Since this location is generally quite impractical, the next best thing is to provide a circuit which will faithfully reproduce across the transient plates of the CRO the voltage which appears across the low side of the potential divider, without introducing any extraneous voltages from any other source. It is believed that the doubly shielded isolated screen room about to be described meets this requirement.The principal elements of this installation, as shown in Fig. 1, include the impulse generator, the wave-forming network, the test specimen, the potential divider, the triple-shielded coaxial transient cable, the doubly shielded screen room, the i...