Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/104

Rh of the tube; about four feet from the firing point, the flame broke a piece of silver foil stretched across the tube, the moment of rupture being marked on a moving plate by an electro-magnetic style. On reaching the further end of the tube, the flame broke a second strip of silver, the moment of rupture being recorded by a second style. To eliminate errors due to retardation of the electro-magnets, a blank experiment was made inmediately before firing the gases. The plate of the chronograph passed over two breaks, which could be adjusted so as to be broken by it at the same instant. In the preliminary experiment, the circuits of the two magnets were completed through these two breaks, and were thus broken simultaneously, the two styles registering their marks on the moving plate. The magnets, without their position being altered, were then connected with the silver bridges across the explosion tube, and the gases were fired by the moving plate striking one or other of the two breaks. The flame travelling down the tube broke the two silver bridges in turn, releasing the two styles, which again registered their marks on the moving plate. The two marks made by the first style gave the interval of time between the spark and the breaking of the first bridge, independently of the error of the electro-magnet; for both marks were equally affected by any error of retardation. In the same way, the two marks made by the second style gave the interval of time between the spark and the breaking of the second bridge, independently of the error of the electro-magnet. The difference between the intervals was the time taken by the flame to travel between the two bridges.

This method of cutting out the errors of the chronograph is only valid when the current flowing through the electro-magnets is constant. A small resistance coil was interposed in the preliminary experiment in each chronograph circuit, to match the resistance of the silver-bridge and connections used in the actual determination. The current was supplied by a storage cell ( and ) to each electro-magnet, and the time between the preliminary and the final experiment was never more than one minute. In all the later portion of the work a confirmatory experiment was made after the gases were fired, the connections being made as in the preliminary. In most cases the styles passed absolutely over the preliminary marks—merely deepening the lines, thus showing that nothing had been displaced. In some cases the preliminary and confirmatory marks were just visibly separated; in these cases the mean position of the two was used to calculate the rate. In the few instances in which the confirmatory did not closely agree with the preliminary marks the experiment was rejected. In order further to eliminate errors of the instrument a second experiment was always made as soon as possible after the first, the connections of the chronograph being all reversed—the first style being connected with the second bridge, and vice versa. The mean result of the two experiments is taken as one determination.

The chief differences between my apparatus and that employed by were the following:—