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27. Kinnersley, Harris and Riess’s Experiments. Spark. Pressure Produced by. Ganot, § 790, et al. Encyclo. Brit. Art. Elect.—These experimenters passed a spark through air contained over mercury, so that if the pressure of the air were increased, the mercury would move along through a capillary tube, having a scale so that the amount could be represented to the eye, as in the cut. (Fig. V.) The experiments proved that when a spark passes through the air, the pressure is increased, and it was concluded in view of several experiments, that the spark being the source of an intense, but small amount of heat, expanded the air, thereby causing the pressure in a secondary manner, through the agency of heat. A spark as short as 2 mm. will produce a considerable pressure of the mercury. Riess performed an experiment also in causing the spark to pass through cardboard, and also through mica located within the air chamber. ssss1. Other things being equal, the increase of temperature was less by using the solid material like mica or cards, than without. This illustrated that a part of the energy of the spark was converted into heat and a part into mechanical force, and explained why sound, ssss1, is produced by a spark and by lightning.