In all cases of transmission of electricity over long distances for lighting or power purposes the currents are "alternating." They flow first one way and then the other, reversing perhaps twenty times a second, or it may be two hundred, or even more times in that short period. Some electric railways are worked with alternating current, and it is used for lighting quite as much as direct current and is equally satisfactory. In wireless telegraphy it is essential. In that case, however, the reversals may take place _millions_ of times per second. Consequently, to distinguish the comparatively slowly changing currents of a "frequency" or "periodicity" of a few hundreds per second from these much more rapid ones, the latter are more often spoken of as electrical oscillations. And these alternating and oscillating currents need to be measured just as the direct currents do. Yet in many cases the same instruments will not answer. There has therefore grown up a class of wonderful measuring instruments specially designed for this purpose, by which not only does the station engineer know what his alternating current dynamos are doing, but the wireless operator can tell what is happening in his apparatus, the investigator can probe the subtleties of the currents which he is working with, and apparatus for all purposes can be designed and worked with a system and reason which would be impossible but for the possibility of being able to measure the behaviour of the subtle current under all conditions. One trouble in connection with measuring these alternating currents is that they are very reluctant to pass through a coil. One method by which this difficulty can be overcome has been mentioned incidentally already. I refer to the heating of a wire through which current is passing. This is just the same whether the current be alternating or direct. One of the simplest instruments of this class has been appropriated by the Germans, who have named it the "Reiss Electrical Thermometer," although it was really invented nearly a century ago by Sir William Snow Harris. It consists of a glass bulb on one end of a glass tube. The current is passed through a fine wire inside this bulb, and as the wire becomes heated it expands the air inside the bulb. This expansion moves a little globule of mercury which lies in the tube, and which forms the pointer or indicator by which the instrument is read. As the temperature of the wire rises the mercury is forced away from the bulb, as the temperature falls it returns. And as the temperature is varied by the passage of the current, so the movement of the mercury is a measure of the current.