Fuses have been treated at such length in Guide No. 2, Chapter XXV, that very little can be said here, without repetition.
[Illustration: FIG. 2,238.--Sectional view of Noark 250 volt, 400 ampere ~enclosed fuse~. The ~fusible element~ _is divided into strips_ A, B, C, and D. This parallel link construction results, upon the operation of the fuse, in the formation of a number of small arcs, thus facilitating the absorption of the metal vapor formed when the fuse blows. ~The fusible strips~, of which there are two or four in number, according to the ampere capacity of the fuse, are entirely surrounded by a granular material which is chemically inactive with respect to the fusible link and whose function is to absorb the metallic vapor formed upon the blowing of the fuse. ~The contact blades~ T and L are made of round edge copper, the round edges facilitating the insertion of the fuses in the circuit terminals. R and S are the end ferrules, attached to cover E, by the pin M.]
~Ques. What effect have the terminals on a fuse?~
Ans. The current at which a fuse melts may be greatly changed by the size and shape of the terminals.
If near together and large, they may conduct considerable heat from the fuse thus increasing the current required to blow the fuse.
~Ques. What is the objection to large fuses?~
Ans. The discharge of molten metal when the fuse blows is a source of danger.
~Ques. What should be used in place of large fuses?~
~Ques. What are the objections to fuses in general?~
Ans. The uncertainty as to the current required to blow them; the constant expansion and contraction is liable to loosen the terminal screws when screws are used.
Ans. They form an inexpensive means of protecting small circuits.
[Illustration: FIG. 2,239.--Cross section through plug fuse. With this type of fuse it is impossible to place any except the correct size of plug in the socket.]
Ans. It is constructed as shown in fig. 2,239, the fuse wire being visible and stretching between the two metal portions of the plug.
Ans. One similar to a simple fuse, but provided with clip contacts as used for knife switch contacts.
The fuse wire is usually contained in a china or porcelain tube, which also serves the purpose of a handle for withdrawing the fuse.