68 



♦ KNOWLEDGE • 



[July 25, 1884. 



eyes followed me. I then returned to my first position, 

 moving more slowly. The eyes still followed for a very 

 brief moment, and then suddenly left me. The face, at 

 this instant, expressed manifest anguish — the poignant 

 agony of a person suffering acute asphyxia. The mouth 

 opened visibly, as if struggling for air ; the head then lost 

 its balance and rolled on one side, the contraction of the 

 maxillary muscles being the last sign of life. From fifteen 

 to twenty seconds had expired." 



M. Petitgand considers that when a head is separated 

 from a bod}', and the loss of blood does not exceed certain 

 limits, and it contains suflioient oxygen to stimulate the 

 nervous function, sensation may last for possibly half a 

 minute. Thus, in certain cases, beheading may be a bar- 

 barous punishment, and " the patient may be conscious 

 some time after his execution ; but, in a great majority of 

 cases, fears of such a survival are chimerical. It is almost 

 impossible that the vertebral column, struck obliquely by 

 the knife of the guillotine, should not occasion a shock 

 sufficient to suspend all the cerebral functions." In the 

 case of the Annamite, the heart made twelve or fifteen pul- 

 sations after the severance of the head. 



It may be concluded from these observations of M. 

 Petitgand that the hanging of criminals in the manner now 

 practised in this country must produce immediate insen- 

 sibility, as the dislocation of the spine must occasion the 

 violent shock which he believes instantly paralyses the 

 nervous system. S. 



THE ELECTRO-MAGNET. 



By W. Slingo. 

 (Continued from p. 30.) 



A LITTLE careful study of the two small diagrams 

 (Figs. 1 and 2) which ajipeared in the previous 

 article (and which are here reproduced for the sake of 

 reference) will have made clear the diotinction between a 



Figa. 1 and 2. 



right and a left handed helix. The idea is perhaps more 

 easily grasped by a reference to the accompanying addi- 

 tional illustration (Fig. 3), where both modes of winding 



Fig. 3. 



are applied to a single coil. The winding commencing at 

 N, the first half of the coil, NS, is a left-handed helix, while 

 the second half, S N', is a right-handed helix. It will be 

 seen that at S the wire is turned back on itself, and the 

 direction of winding reversed. 



Now for the polarity induced. The arrows in each case 

 indicate the direction taken by the current. Thus in 

 Fig. 1 the current enters at the end marked N, and passes 

 round the glass in the opposite direction to that taken by 

 the hands of a clock. Kemembering, however, what was 



said about the little man swimming with the current, that 

 is, that when facing the iron or the axis of the coil, the 

 north pole would be on his left hand, we can easily deter- 

 mine the polarity of the ht-li.ic and the iion embraced by it. 

 Thus, if we imagine the little man to be in the upper 

 portion of the first ring or loop, he must, to follow out the 

 conditions referred to, lie face downward with his head in 

 the direction of the retreating current (or away from the 

 reader's eye). Clearly his left hand will be towards X, and 

 his right hand towards S. Supposing the miniature man to be 

 in any other portion of the ring, he willstill have his left hand 

 N-wards, and consequently every ring or every fraction of 

 a ring helps to increase the intensity of magnetisation. It 

 may, however, be observed that the rings are not perfect, 

 but that in passing from one ring to the next there is a 

 slight longitudinal deviation. The sum total of these devia- 

 tions, s'lpposing the iron rod to be covered from end to end, 

 is equal, evidently, to a single wire the length of the coil, or 

 of its iron core. The effect of such a wire would be exerted 

 at right angles to that of the series of rings ; but inasmuch 

 as the single wire Ls materially less etTectual than the rings, 

 and for other reasons which need not here be dwelt upon, 

 such an antagonistic tendency as that exerted by the wire 

 is so infinitessimal as to be lost sight of. Thus, then, in a 

 left-handed helix the end where the current enters becomes 

 a north pole, the other end becoming a south pole. 



Conversely it will be seen that in Fig. 2, where there Ls 

 illustrated a right-handed helix, the arrows indicate that 

 the current pursues the same direction as the watch-hands 

 (viewed from the end at which the current enters), and 

 that to comply with Ampere's conditions the little man in the 

 upper portion of any loop or ring must lie face downwards, 

 with his head towards the reader, when his left hand will 

 be extended towards the extremity of the iron rod marked 

 N, or in the opposite direction to that indicated in the 

 previous figure. The polarity of the iron rod is therefore 

 reversed, and it may be well to note that with a right- 

 handed helix the point of entry becomes a south pole, the 

 other end becoming consequently a north pole. 



But now the interesting question presents itself : What 

 is the efiiect of changing the heliacal direction, or the 

 direction of winding around one and the same core 1 To 

 take the simplest case, we may place a left and a right- 

 handed helix end to end on a common core, and then, joining 

 them together, we shall get a result resembling the accom- 

 panying illustration (Fig. 3). Here the current enters a 

 left-handed helix at N, but at S it leaves the lefthanded 

 and enters a right-handed helix, through which it passes, 

 emerging at N'. In this case, the coil and its core have 

 three poles, one south and two norths, and a little reflection 

 or re-perusal of what has been previously said will make 

 clear the appearance of a south polarity at S, due in the 

 first place to the current emerging from a left-handed helix, 

 and strengthened by the current entering a right-handed 

 helix. In both cases the tendency is to create this south 

 pole. Similarly, the two extremities of the core become 

 north poles on account of the current at N entering a left- 

 handed helix, and at N' leaving a right-handed helix. It 

 may be as well to point out here that, although we may 

 maintain that the two magnetic conditions are equal in 

 every magnet, it does not follow that every magnet contains 

 or possesses only two poles at the extremities separated in 

 the centre by a neutral line or zone. This idea is more 

 prevalent than most people imagine, and it is on this 

 account that the point is here referred to. We may 

 introduce as many extra or " consequent " poles (as they 

 are styled) as we please, bearing in mind, however, that 

 every two north or every two south poles will be separated 

 by a south or a north pole, as the case may be, and that 



