If a large number of charges are to be fired simultaneously with a safety fuse, the length of the fuse to each charge should be made equal or a safety fuse used to a common center and approximately equal lengths of instantaneous fuse or Cordeau Bickford used from there to the charge. In splicing the fuses for such connections they are cut diagonally as shown in Fig. 123 and bound together firmly with tape. Electric connections are particularly advantageous under such conditions as they avoid the dangers incidental to spliced fuses and are less expensive. In tunnel work simultaneous electric detonation is not desirable as the holes should be fired progressively: 1st, the cuts; 2nd, the relievers; 3rd, the backs; 4th, the sides; and 5th, the lifters. Different lengths of safety fuse, or delayed action electric fuses can be used for these delay shots. In igniting a safety fuse an open flame such as that furnished by a match or candle is the most satisfactory. For electric fuses the current is generated by a magneto shown in Fig. 120. Pressing vigorously down on the handle closes the circuit and generates an electric current which heats the platinum bridges and explodes the charges. For the small number of charges used in ordinary construction they are connected in series so that if there is a broken connection anywhere no charge will be exploded. If many charges are to be fired and a line circuit is to be used, the final connection should not be made until just before the charge is to be fired in order to obviate the danger of stray currents firing the charge prematurely. Care should be taken to see that all connections are good and that there are no broken wires on the line. =178. Quantity of Explosive.=—The quantity of explosive to be used can be determined satisfactorily only by experience on the job in question, as the factors affecting the necessary quantity are so diverse. The figures in Table 64 indicate the relative amounts needed under different conditions. TABLE 64 QUANTITIES OF EXPLOSIVES