302 



♦ KNOWLEDGE 



[April 10, 1885. 



water. Now, it does not appear to me likely that these, or 

 most of them, are produced by the collision of the falling 

 drops — for the paths of two neighbouring drops must be 

 parallel, since the drops are subjected to precisely the same 

 get of influence?. 



I believe the phenomenon to be one worthy of more 

 careful notice than it has received — in fact, I am not aware 

 that it has been noticed at all. The motions of the par- 

 ticles are themselves interesting — seeming almost as inde- 

 pendent of gravitation, wind-currents, or the like, as the 

 motion of a flight of insects would be. It is hardly neces- 

 sary to observe that if these particles show that rain is 

 being generated in the lower as well as the upper strata of 

 the air, all difiiculty in explaining the results of Professor 

 Phillip's observations vanishes at once. 



THE YOUNG ELECTRICIAN. 



By W. Slikgo. 



(Continued from p. 232.) 

 SCREWS. 



IN every branch of mechanical industry, screws of 

 various shapes and sizes take a most prominent part, 

 and a knowledge of their functions and properties is 

 essential to one who aspires to any degree of efiiciency in 

 electrical instrument making. Let us, then, turn our 

 attention to what may be regarded as the mainstay of 

 almost every piece of apparatus — good screws, 



Ex. XLII. — The primary function of a screw is to 

 afford a ready means of attaching two pieces of metal, 

 wood, io. Were this the only oVjject, however, simpler 

 and cheaper methods could be resorted to, such as rivet- 

 ting (in the case of metals), or nailing (in the case of 

 wood). A screw is superior to either of these latter pro- 

 cesses in more ways than one. Briefly, these superiorities 

 may be thus enumerated. 



{a) The parts fastened together by means of a screw are, 

 or should be, always detachable. 



(b) The parts so fastened are held together more firmly 

 than is, generally speaking, possible when either of the 

 other modes of attachment is adopted. Of course, nothing 

 in the way of attachment can excel the rivet when the 

 hole is made perfectly round, the rivet perfectly fitted and 

 driven in hot, the head being also made while the metal 

 is in that condition ; then an almost solid mass results. 

 But with such work a.s this we shall have practically 

 nothing to do, and we may confine ourselves, therefore, to 

 the question of screws. 



(c) Screws, when driven home, have the, to some extent, 

 exceptional power or property of drawing the parts firmly 

 together. 



The features essential to the character of a good screw 

 are : — 



(a) Suflicient strength must be possessed by the screw to 

 enable it to draw the parts together without stripping the 

 thread, and to withstand such strains as may be exerted 

 upon either of the parts. 



(6) The thread mus-t be sufliciently substantial to allow 

 of the screw being driven and withdrawn a number of 

 times without impairing it. 



E.\. XLIII. — Screws may be divided into two classes : — 



(a) Those intended for wood-work, and 



(b) Those intended for metal-work. 



Fig. 17a illustrates the universally-known wood screw, 

 in which the head is finished for a coimter-sunk hole. 



'Vv 



Fig. 17 



Another form of wood screw is depicted 

 in Fig. 176, in which the underside of the 

 head is flat. The thread is thin, and the 

 distance between the threads (called the 

 "pitch") comparatively great, in order 

 that the thread formed in the wood shall 

 be approximately equal in strength to the 

 thin thread on the screw. 



There are many varieties of metal screws. 

 They are named according to the number of threads cut in 

 each inch of metal. So far as the threads are concerned, they 

 generally take one or other of two forms, viz , the square 

 and the V-shaped. With the former, however, we shall 

 have little or nothing to do. Let us, therefore, direct our 

 attention to the other class, which we shall have frequent 

 occasion to use. The best class of Y-shaped thread for 

 heavy work is doubtless that known as the Whitworth, 

 with which we will deal presently. 



The young electrician will, as a rule, require only small 

 screws, not made after the Whitworth pattern. The thread 

 is comparatively fine, and each maker has his own parti- 

 cular pattern. There are naturally several sizes suitable to 

 the various requirements. One feature pertaining to metal 



Fig. 18. 



screws is the variation in the head. Fig. 18 illustrates a 

 number of screws in which heads of different sizes and 

 shapes are employed. It will be noticed that some are 

 long and narrow, specially applicable when the head has to 

 be let into a hole, while others are wide and short The 

 latter are useful when it is desired to cover up a small hole. 

 The screws illustrated are of various sizes, but to similar 

 sized heads screws of many different dimensions are 

 procurable. 



THE .SCREW-PLATE. 



Ex. XLIV. — The next tool claiming our attention is 

 the screw-plate. Fig. 19, which consists of a thin sheet of 

 hardened steel containing a number of screw-holes provided 

 with cutting edges formed by smaller holes drilled at the 

 extremities of a diameter of the screw-hole to as to cut 

 away portions of the thread. The function of the screw- 

 plate is to enable one to make screws ranging in size up 

 to about an eighth of an inch in diameter, and to make 

 other tools known as " taps," the use for which is to pre- 

 pare a thread in the piece of metal which is intended to 

 receive the screw. Obviously, then, the screw-holes in the 

 plate are of various sizes, although the thread is in each 

 case identical. The drilled holes which provide the cutting 

 edges also afford means of exit for the metal shavings cut 

 from the piece of wire, (fee, upon'which the thread is being 

 produced. 



SCREW-MAKIN'Gr 



Ex. XLV. — There is, of course, a right and a wrong 

 way of using a screw-plate. Let it be supposed that we 

 wish by its means to make a screw. Having determined 



