42. POGGENDORFF’S EXPERIMENT. EFFECTS OF INTERRUPTING A CURRENT WITHIN DISCHARGE TUBE. _Phil. Mag._, 4th Se., vol. x., 1855, p. 203-207.—Imagine an electric bell vibrator and magnet within the glass receiver upon an air pump. Upon connecting the magnet and vibrator in series with a small electric battery, it is evident that in the open air, as usual in electric bells, there will be a minute violet spark at the terminals of the circuit breaker. § 6. Now, let the air be exhausted as far as possible by means of a mechanical pump as constructed in 1855. Poggendorff performed such an experiment, and he noticed that in the poor vacuum the ordinary violet spark became yellow, while blue light like a small enveloping tube surrounded the hammer of the vibrator and wire leading to the opposite contact and a little projection extending away from the hammer. His experiment was unique, because showing for the first time that a current from a battery, if interrupted in the vacuum, will not only produce the usual minute spark, but that a blue tube of light will be produced around the conductors within the vacuum. 43. DE LA RUE AND MÜLLER’S EXPERIMENT. SOURCE OF THE STRIAE AT THE ANODE. NUMBER OF STRIAE VARIED BY CHANGE OF CURRENT. _Phil. Trans._, 1878.—By an arrangement of means for causing different pressures, they made a discovery, namely, that as far as the eye is concerned the striae begin to have their existence at the anode. § 46. Imagine the internal gas pressure to become less and less. First, a violet luminosity occurs around the anode as in § 42. As the pressure becomes less and less, luminous striae move toward the cathode accompanied by more and more striae, which increase either to form a column reaching a certain distance or else extending through the whole distance between the electrodes. § 46. They observed that when the E. M. F. was constant and the current changed, the variation in the appearance of the striae was very regular. § 41. With some tubes the number of striae increased with the increase of current, while with a decrease of current the number of striae became less and less. § 8_a_. With some tubes the number of striae increased while the current decreased. § 8_a_. With the use of a condenser, then as the E. M. F. decreased together with a diminution of current, the number of striae varied. The striae nearest the anode vanished first, as they diminished in number with the fall of the E. M. F. The striae on the other hand originated at the anode, when the charge of the condenser was gradually increased from a minimum, and then the striae continued to increase from the anode as the source. As to the color of the striae, the same was changed by an alteration of the current.