HISTORY AND PROGRESS. 213 



c and c', with adjusting screws. Opposite these points 

 are two similar ones, projecting from the terminals A 

 and A', which are connected by the spirals w and w with 

 B and B', respectively. On the margin of the board are 

 five terminals for receiving the wires of lines, apparatus, and 

 earth ; the bar c is connected by a wire underneath the 

 board, shown by dotted lines in the figure, with the back 

 terminal E, destined for the earth connection ; the terminal 

 A is similarly connected with a on the one side, and A' with 

 ' on the other ; B and B are also in permanent connection 

 with b and b on opposite sides. The two line wires are 

 brought to a and a, and the apparatus is inserted between 

 B and B'. 



114. Kerekkoj/T* Lightning Discharger includes arrange- 

 ments for discharging the line both between surfaces and 

 points. A hollow brass cylinder, supported by a bracket, is 

 in permanent connection with earth. Inside the cylinder 

 is a second metal cylinder, insulated from it by short ivory 

 tubes at the ends, and held in its place by the screw-points. 

 The annular space between the cylinders does not exceed 

 one- fourth of a line, so that electricity of moderately low 

 tension can easily spring over. The line wire is connected 

 with one of the two terminal screws of the inner cylinder, 

 and the wire leading to the apparatus with the other. The 

 outer extremities of the screws are furnished with points 

 by which, should the electricity fail to leap over the annular 

 space between the cylinders, it will in all probability dis- 

 charge itself to the opposite points on two screws connected 

 to earth through their uprights and the common base-plate. 



115. Bianchi's Vacuum Lightning Discharger. Du Moncel 

 describes this apparatus, which is of a novel but somewhat 

 inconvenient construction. It consists of a brass globe 

 inserted in the line circuit, covered and protected by two 

 hemispheres of glass cemented into a broad metal ring, 

 which is provided with radial spikes pointing inwards to 

 within a very short distance of the surface of the globe. 

 The latter is held in its place in the centre by an axis 

 passing air-tight through the poles of the glass hemispheres 



