SEISMIC METHODS 



861 



time-break is satisfactory provided the interruption of the firing current 

 and the detonation of the cap are simultaneous. This is not always true, 

 however, of all caps. In general, two types of electric blasting caps are 

 offered by explosives manufacturers for seismograph use. One is the 

 type of cap used in industrial blasting. Typical firing characteristics of 

 this "regular" type of firing cap are illustrated in Figure 536 by curves 

 No. 1 and No. 1-A. For current values of about two amperes or less, the 

 rupture of the bridge wire is simultaneous with the detonation of the cap. 

 Above this current value, however, the interruption of the bridge wire 



FIRING CURRENT - AMPERES 



Fig. 536. — Curves showing firing characteristics of the typical 

 "Regular" electric blasting caps (industrial type) and special 

 "No-Lag" seismograph caps. 



circuit precedes the detonation of the cap by a time interval dependent on 

 the firing current employed. These "regular" electric blasting caps are 

 widely used for seismograph work and are found entirely satisfactory 

 where low firing currents are used or where firing conditions are such that 

 a correction can be made for the time lag between the breaking of the 

 bridge wire and the detonation of the cap. 



There are circuits, however, in which these conditions cannot be main- 

 tained, and some explosives manufacturers furnish caps in which the in- 

 terruption of the cap current is practically simultaneous with the explosion 

 of the cap regardless of the firing current used. In these caps the bridge 

 wire does not fuse and is therefore not ruptured until the cap explodes. 

 In general, the "no-lag" caps fire more rapidly at any current than do the 

 "regular" caps. Typical firing characteristics of these special seismograph 

 caps are illustrated in Figure 536, curve No. 2. 



Electric blasting caps are usually furnished in two strengths : the No. 

 6 and No. 8 grades. When fulminate of mercury was the universally 

 used detonating material these strengths were well defined, the No. 6 caps 



