Brown. — Action of Fusible Cutouts. 361 



able effect of shellac upon fuse-wires, which was mentioned 

 by Sir W. Preece. I have not yet fully examined the effect 

 on the fusing-current ; with a 100-mil wire, long, it increases 

 the fusing-current about 14 per cent., and 10 per cent, with 

 a 40-mil wire. I may here note that thorough oxidation of 

 the surface has the same effect (about) as shellac upon the 

 100-mil wire. 



If the plain fuse-wire simply stretches betvveen its ter- 

 minals the effect of fusion is first noticed by the centre melting 

 and dropping into a catenary, which slowly increases in length 

 until it reaches within, say, iin. of the terminals (if the fuse 

 is short), when it attains a dull-red heat (owing, no doubt, to 

 the specific resistance of molten tin being probably about 

 double that of the solid metal). Oxidation sets in, and after 

 a few hours a thin wire will burn right through. If the wire 

 is long enough, the catenary will increase in length till the 

 weight breaks it off. By experiment I have found that this 

 length is roughly represented (for sizes over 20 mils) by 

 the formula — 



L D = 40, where L = length of catenary in inches, and 

 D = diameter of wire in mils. 



40 



Thus a 40-mil wire requires a length of tq = 1 in. for its 



catenary, with, say, iin. allowance at each end — 2 in. in all — 

 before it will break without getting red-hot. So with 20 mils 

 the length required is 3 in., with a clear space of about f in. 

 underneath, to allow for the droop of the catenary. But 

 if the wire is shellacked the effect is that upon fusion the 

 shellac may be seen to dance about in a very lively fashion on 

 the surface of the molten tin, which presents a mirror-surface, 

 until a length of Jin. is fused, when the wire snaps sharply, 

 the molten material flying back and forming little beads on 

 the unmelted end of the wire. (The voltage of the circuit was 

 about 5 volts, alternating.) This observation seems to be 

 of some practical importance, as I think it is generally under- 

 stood that shellacking a fuse-wire is intended merely to pre- 

 vent oxidation. 



I may say that I consider the phenomenon to be due to 

 surface-tension, and not prevention of oxidation, because 

 while the before-mentioned catenary is forming, the tin wire 

 breaks off in little avalanches as it melts into the catenary, 

 the surface presenting momentarily the bright appearance of 

 unoxidized metal. 



I will now give a table of the properties of my materials so 

 far as I have determined them at present. I have to thank 

 Professor T. H. Basterfield for the determination of the specific 

 gravities of the materials, an operation requiring great skill 

 and care to insure accuracy. I have applied the corrections 



