490 



MAGNETO-ELECTRICITY. 



intense light known, it was naturally looked to 

 as the possible means of perfecting the whole 

 system. But the light produced by electricity 

 to be applicable for lighthouses must be certain 

 and constant, not liable to extinctions or any 

 great variations, as the first would tend to en- 

 danger vessels seeking and not finding the 

 light ; and if a fixed light had much variation, 

 it might be mistaken for a revolving light. 



Frictional electricity will give a succession 

 of flashes intensely vivid, and might be used 

 for the purpose, but for the fact that the slight- 

 est moisture is sufficient to convey the whole 

 charge to the earth. The various forms of 

 galvanic battery are all capable of producing a 

 steady and intense light, but still (besides the 

 great expense) they are not applicable, because 

 of the necessarily varying current, which be- 

 comes weaker and weaker as the solution 

 becomes saturated. The magneto-electric ma- 

 chine is then the source from which one would 

 naturally expect a light which should be invari- 

 able in its nature and capable of being contin- 

 uous for any given time, as the current produced 

 by this machine is constant as long as the heli- 

 ces revolve with the same speed, and the speed 

 can be easily regulated to any required velocity. 



The electricity derived from a magneto-ma- 

 chine is induced in coils of wire, by the chang- 

 ing of the magnetic polarity of pieces of soft 

 iron inclosed within the coils or helices; and 

 the quantity or intensity of the induced current 

 depends, first, on the amount of magnetism in- 

 duced in the soft iron ; secondly, on the facility 

 with which the poles of the magnetized soft 

 iron c:m be reversed ; thirdly, on the velocity 

 with which the change of polarity takes place; 

 fourthly, on the length and diameter of the 

 wire forming the helices. 



The amount of magnetism induced in the 

 soft iron depends on the size and force of the 

 steel magnets employed, and on the weight 

 and softness of the iron in the helices ; but the 

 weight in practice of the soft iron is limited by 

 the weight of the steel magnets, for, if too 

 heavy the steel magnets will be slowly deprived 

 of their magnetism. To facilitate the change 

 of the poles the soft iron cores of the helices 

 are not solid pieces of iron, but are tubes, sin- 

 gle, double, or treble, as it is found by experi- 

 ment that the same weight of iron, when divid- 

 ed in this manner, loses or takes magnetism in 

 much less time than when in a solid form. 



There is a limit to the velocity to be employ- 

 ed when the maximum of electricity is re- 

 quired, for this reason : It has been already 

 remarked that the amount of electricity de- 

 pends on the amount of magnetism taken up, 

 and that the soft iron takes time to become 

 saturated, as it may be termed, with magnet- 

 ism ; hence, if the velocity be too great with 

 which the cores move from one pole of a mag- 

 net to another, there will not be sufficient time 

 for the cores to become saturated. But as 

 again the quantity of electricity increases as 

 the velocity increases, it is necessary to ascer- 



tain this maximum point exactly, which u 

 easily done, either by experiment or calcula- 

 tion, based on certain data. The length and 

 diameter of the wire require to be different, 

 according to the current required ; for a short 

 thick wire forming the helices represents a 

 galvanic battery composed of a dozen, say, of 

 verr large pairs of plates, whilst a long thin 

 wire would represent a battery composed of 

 thousands of small plates. In other words, 

 supposing the size of the helices to remain the 

 same, if they are composed of thick short wires, 

 quantity is obtained; but if composed of long 

 thin wires, intensity will be the result. 



From all this it results that there are certain 

 laws known and established, by which a mag- 

 neto-electric machine can be made to give a 

 current of any given amount of electricity, with 

 any given ratio between its quantity and inten- 

 sity. 



Having seen on what the production of the 

 current depends, the next point to observe is, 

 the peculiar nature of this induced current. It 

 differs essentially from a galvanic current in 

 this, that while the helices are revolving, the 

 direction of the current is reversed, as the core 

 of soft iron passes each consecutive pole of the 

 steel magnets. 



It now remains to explain how the current 

 generated in the wires of the helices is to be 

 withdrawn from the machine. In the first 

 place all the helices are connected in two, or 

 four, or more series, and in doing this great 

 care must be observed that the direction of the 

 coil of every alternate helix is in an opposite 

 direction ; that is, if one is wound as a right- 

 hand screw, or, what amounts to the same 

 thing, supposing all wound in the same direc- 

 tion, then the two inner ends of the wires must 

 be joined of, say, numbers one and two, and 

 the two outer ends of the wires of numbers 

 two and three, and so on through the series ; 

 and lastly, the terminals of the series might be 

 soldered into two insulated disks, and then led 

 from the machine by two pieces of metal kept 

 in contact with the outer surfaces of these disks 

 by a slight spring ; such an arrangement allows 

 the alternating current to pass from the ma- 

 chine, and such a current will produce a light, 

 but this light has certain disadvantages. It is 

 never white, but always more or less blue or 

 brownish; in fact it is like the electric light 

 obscured by placing it behind a flame from 

 spirits of wine. It is also extremely injurious 

 to the eyes, both from its color and from its 

 tremulousness. Mr. Holmes therefore does not 

 use this current, but in its stead converts this 

 constantly inverting current into two that flow 

 from the machine in one direction only. This 

 is accomplished thus: One-half of the helices 

 are arranged so as to arrive on the poles of the 

 magnet at the instant that the other half are 

 exactly midway between the poles. Thus there 

 are two distinct currents ; and what may be 

 called the dead point, that is the point when 

 the current inverts in one series, occurs exact- 



