678 mene 
firing line be isolated from the other cireuit grounds. A firing pulse can, 
in this case, be obtained from the firing eircuit by inserting an isolation 
transformer in series with one side of the firing line, the trigger pulse 
being taken from the transformer secondary, one side of which may then be 
grouaded, 
Another method is to use a switch mechanically coupled to the firing 
switch but electrically independent of it. Any switch will have a time 
difference between closing of the two circuits, and in many switches this 
interval is of the order of milliseconds and not very reproducible, It has 
been found that some ordinary a-c tumbler switches as used in house wiring 
give a time difference between the two circuits of less than 100 psec, which 
has been found reproducible to less than 10 psec. 
Another means of synchronization utilizes the change in firing-line 
current when the circuit is broken by detonation of the blasting cap, Sever- 
al practical problems arise in application of this method, First, the cir- 
cuit must be arranged to prevent the possibility of extraneous voltages 
triggering the circuits prematurely. Such signals may be developed by chatter 
in closing of the firing switch or change in firing-circuit resistance due 
to heating of the blasting-cap fuze wire before it burns out. Second, the 
breaking of the blasting-cap circuit is not necessarily coincident with the 
instant that detonation begins, Seismographic caps are especially designed 
to insure this coincidence and can be used successfully in small-charge work. 
A great deal of difficulty was experienced with early attempts to use the 
break of the cap circuit in detonating service weapons. These difficulties 
were never completely explained, but it is suspected that ionization of the 
gas sphere after detonation of the charge gave rise to conduction in the cir- 
cuit which prevented satisfactory operation, 
(ii) Pressure switches. In general, attempts to use the break of the 
firing circuit have given unsatisfactory results. Methods that work properly 
for a while break down when one begins to have confidence in them. As a re= 
sult, this type of synchronization has been abandoned in favor of more de- 
pendable methods. The first of these used at this laboratory was a simple 
switch, consisting merely of a lead disk -- supported at its edge -- which 
was driven against a steel pin by force of the shock wave. This switch func- 
tioned reliably but had the disadvantage of requiring reassembly for each 
