32 G. H. LOVING AND G. H. SMITH 
little or no difference between the detonation rates of the two electric 
blasting caps involved. 
The third time break system is of fairly recent development and 
has proved so efficient that it has replaced, to a great extent, the two 
methods previously mentioned. With this type, only one cap is used, 
this detonator having two functions, first, the detonation of the 
dynamite charge and, second, the production of the desired time 
break. The latter step is accomplished by so arranging the time cir- 
cuit that the leg and bridge wires of the electric blasting cap form a 
part of this hook-up. On this basis, the time signal is given when the 
bridge wire fuses or breaks. 
With the single cap set-up just described, the elapsed period, or 
lag, between the breaking of the bridge wire and the detonation of 
the main charge of the electric blasting cap, is the important time 
value. Tests by the companies engaged in geophysical prospecting 
demonstrated that the lag with standard electric blasting caps was of 
such magnitude as to render the caps unsatisfactory. Laboratory and 
field tests were conducted, therefore, on the development of a de- 
tonator with the necessary qualifications. 
In the laboratory investigation, a cathode-ray oscillograph was 
employed, this instrument being exceptionally well adapted to the 
measurement of small intervals of time. The experiments covered 
numerous designs of electric blasting caps and involved evaluation of 
the characteristics of different types of bridge wire and explosive com- 
pounds. The effect of firing amperage was also determined. This lat- 
ter phase of the program developed some very interesting information 
on the relationship between amperage and time lag. For example, it 
was found that, if a low current, say 0.75 amperes, were used in firing 
any of the types of electric blasting caps now being manufactured, the 
severance of the bridge wire and detonation of the main cap charge 
took place simultaneously. As the amperage was increased, a measur- 
able lag occurred. 
It is to be understood, of course, that the above discussion refers 
to the time lag between bridge wire severance and cap detonation. 
If the lag is taken as applying to the total firing time of the cap, the 
picture is changed entirely. On the latter basis the detonation rate 
of the electric blasting cap was found to become greater as the am- 
perage was increased. 
During the oscillograph tests, there was developed a combination 
of bridge wire and cap charge which gave promise of having desirable 
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