SVK Test Model 4. This model was substantially the same as the thrid, except that the photo-electric cell was replaced by selenium cells and the current fed over a con-denser, without amplification, to the I relay. The selenium cells had a primary potential of three vols. Tests of this model in the Baltic and North Sea were completed with satis-factory results. Firing was obtained from target ships of 1000 gross registered tons at speeds above three knots in depths up to 65 feet.