Some of the simplest and most practical galvanometers, specially designed for ascertaining the actual efficiency of conductors, have been made in recent years in Germany. The author of this work had constructed for him by Mr. H. Yeates, of Covent Garden, the one, with some improvements, as shown in the subjoined engravings: the first, fig. 1, exhibiting the arrangement of the battery and resistance coils, and the second, fig. 2, giving a diagram of the battery current. The battery consists of three cells, and is a modification of the old manganese cell, in which the carbon and oxide of manganese occupy the outer, and the zinc plate the inner, or porous, cell. By this arrangement, the surface of the negative element is greatly increased, and hence a more constant current is obtained, on account of the battery not polarising so rapidly as in the old form. Another advantage of this arrangement is, that the cells can be almost entirely sealed up, the air-openings being made within the porous cell. In the centre of the lid of the box is placed the galvanometer with a ‘tangent’ scale; and on the left are two terminals, by the connection of which the conductor can be examined. On the right hand end of the lid are placed five keys, marked respectively, L, B, 1, 2, 3. Under B is one pole of the battery, so that by depressing this key, as will be seen by the connections in the diagram (fig. 2), the battery current is sent through the galvanometer direct. If, however, key No. 1 is depressed, the battery is connected with the galvanometer through a known resistance--key No. 2 has a larger resistance, and No. 3. still larger. The fifth key, L, closes the circuit within the limit of the instrument, but on being depressed opens it, and includes the line or conductor placed between the two terminals at the other end, the battery key at the same time being pressed down. By this arrangement it will be seen that the resistance of the line or conductor may be compared with the known resistance connected with any of the keys Nos. 1, 2, 3, or any of these resistances may be included with that of the line, so as to get a convenient deflection of the galvanometer needle. In the case, with the battery, is a bobbin of insulated wire for connecting the instrument with the conductor and earth which is to be tested. The whole arrangement here described and illustrated is exceedingly portable, being in the form of a small carpet bag, and therefore particularly fitted for persons inspecting lightning conductors and making periodical tests, without which it cannot be too widely known there is really no trustworthy security of protection in lightning conductors. CHAPTER VI. CHARACTER OF LIGHTNING AND OF THUNDERSTORMS.