Q. Explain the action of the combination electric friction primer.
A. It is a combination of the two above principles, except that in the electric feature there is no return wire in the combination primer. The current comes in through the stem from the copper contacts on the firing mechanisms, heats the platinum, etc., and goes out through the case of the primer to the breech-block of the gun, thence to the trunnion and to the other pole of the firing-battery. This primer is also obturating. (Fig. 38.)
Q. What is an obturating-primer?
A. One which prevents the escape of powder-gas through the vent. (It need not necessarily be screwed into the vent.)
Q. What are percussion-primers?
A. Such as are used in fixed ammunition and go off by the snap of the firing-pin. They are commonly called caps.
Q. What are high- and low-resistance fuses?
A. A high-resistance fuse is one that requires a high initial velocity to cause the plunger to be released, and is therefore safer in handling. A low-resistance fuse does not require such a high initial velocity to cause it to act. Low-resistance fuses are provided with "safety-wires." (See Fig. 39.)
Q. What fuses are used in the United States service?
A. Frankford Arsenal base percussion, point percussion, and combination. The first two are of either high or low resistance, and vary in size for different calibers of guns. All fuses that are not "centrifugal" have become obsolete for all calibres above 1·65", except the combination fuse, but those on hand will be in service until used up.