M 



UNDULATORY FORCES. ELECTRO-MAGNETISM. 



[THE MACHINE. 



electric current. ID this experiment, M in that to which 

 we have alluded, wo shall find that the effect* of the 

 forces are produced at right angles to the direction 

 of each. 



Sxperimrnt 20. Coil nbont one hundred feet of No. 

 39 or 84 covered copper wire, round a paper or gutta- 



md one inch in iliamutor; 

 il to tho binding-screws of a 



galr::. ' Tli. n introduce one end of a rod, or 



bar magnet. suddenly ; and nt the moment this is done, 

 it will DO : it tho noodle of the galvanometer 



diverges. It will,- however, speedily regain its old posi- 

 tion. If tho magnet be now suddenly withdrawn from 

 tho coil, the needle will immediately diverge in an oppo- 

 site direction to that which it previously took. 



This experiment shows that a current of electricity 

 is immediately induced in tho wiro by the magnet, on 

 its introduction into, and withdrawal from, the coil ; 

 and, further, that the effects aro produced only when 

 these movements take place ; for no result follows if tlw 

 magnet 1* kept permanently in the coil after its intro- 

 duction is effected. This is highly analogous to the 

 phenomena of secondary electro-induction, fur wo notice 

 that tho current was produced in the secondary coil, only 

 wheu contact was made and broken. This fact, which 

 was first discovered by Dr. Faraday, is of the most 

 interesting kind ; it establishes a chain of analogies be- 

 tween each branch of electrical science, and that of 

 magnetism ; it opens out a wide field for the exercise of 

 inductive generalisation, and establishes a definite con- 

 nection between the causes which tend to produce nil 

 tin- varieties of electric and magnetic disturbances, and 

 other phenomena, whether as witnessed in our labora- 

 tories, or in the more magnificent and boundless one of 

 nature. 



The experiment which wo have named will be a suffi- 

 introduction to our subject, inasmuch as the stu- 

 dent can easily repeat it, and personally ascertain tho 

 results wo hare called attention to. Wo shall, there- 

 fore, proceed '.to describe the K-st forms of magneto- 

 electric machines, such as are employed at tho pi- 

 day. The simplest of the kind is that sold at the 

 instrument-makers, as an American invention, for the 

 puqiose of giving shocks. It consists of a single horse- 

 shoe permanent magnet; opposite to which aro two small 

 rods of iron, which revolve close to the poles of the 

 magnet being turned on their axis by the ordinary mul- 

 tiplying-wheel arrangement. In this instrument the 

 magnet is not inserted in the coils, but the iron in them 

 becomes magnetic by induction, as it passes either pole 

 of tho magnet. If these rods, or armatures, as they 

 are called, were made of steel, they would become per- 

 manently magnetic'; but being of soft iron, they instantly 

 loe their magnetism on getting beyond the range of 

 induction ; and tlieir rapid revolution and consequent suc- 

 cessive appearance before, and on leaving tho permanent 

 magnet, ans-.ver exactly tho same purpose as that of 

 inserting and removing the bar magnet, aa explained in 

 our elementary experiment. These armatures are covered 

 with a great length of fino covered copper wiro, which is 

 roiled in a similar manner to that wo have already de- 

 scribed in connection with various electro-magnetic in- 

 struments. Two sets of armatures are provided in large 

 i ', of thick wiro, for tho purpose of 

 M of quantity (light and neat) tho 

 ployed for various effects of intensity, 

 such as giving of shocks, <tc. Tho en<ls of these wires 

 aro attached to a contact-breaker, which consists of 

 alternate pieces of wood and brass, against which two 

 springs rest. From these springs the two wires proceed, 

 elided by binding-screws ; and tho various experiments 



i 



are conducted by means of other wires attached to those 



:!. ML 



Tho larger kinds of those machines have had chiefly 

 two forms ; in one of which tho armature* revolve at the 

 ends, and, in tho other, at the face of tho poles of the 

 magnet ; and, in such, the magnet consists of a nn: 

 of the horse-shoe kind, fonning a compound an. 

 ment of groat power. The following engraving illus- 

 trates the appearance of ono of those instruments. 



Fif. M. 



Tho power of these machines depends on tho size and 

 number of tlie magnets; the di>tauce of the armatures 

 from them; tho length and thickness of the wire wound 

 on tho armatures; nnd the rapidity with which i 

 aro made to revolve in face of tho magnets. The form 

 of instrument used for the purposes of electro-plating 

 lias been already illustrated and described ;t and as its 

 several parts are the same, in principle, in all instru- 

 ments, although they vary in form, wo shall refer our 

 readers to what we have already stated on this part of 

 the subject. 



Mr. Holmes, who has been very successful in apply- 

 ing magneto-electricity for the purpose of obtaining the 

 electric light, uses a very ditlbrent arrangement to any 

 we have described. His armatures are fixed with 

 their coils on a cylinder; over these the magneto aro 

 made to revolve face to face ; and they are driven by 

 steam power. The appearance of his machine is repre- 

 sented in the folio plate. It has been engraved to illus- 

 rated one which was shown in operation at the Inter- 

 national Exhibition of 18C2. To the left of the cut an or- 

 dinary electric lamp is seen ; and on the right, a polyzonal 

 lens, such as was employed for some time at the South 

 Foreland lighthouse, with one of these engines, in place of 

 the ordinary oil lamps, which it far exceeded, in tho clear- 

 ness and brilliancy of its illuminating power. 



It will bo unnecessary for us to suggest any exi 

 roents with tho magneto-electric machine. It will 1 c 

 Miillicient for us to state, that all those wo have men- 

 tioned under the head of Voltaic Electricity, Electro- 

 Metallurgy, and Electro Ma, 'net ism, may bo as readily 

 performed by it as by voltaic batteries ; tho success 

 brilliancy of course depending on tho sizo of the instru- 

 ment. Its chief use, at tho present time, is for electro- 

 plating, it. ! purposes ; and, to a limited exi 

 as a substitute for tho voltaic battery in electro-tele- 

 graphy. Tliis application we shall enter into in our 

 next chapter. 



As a philosophical instrument, the magneto-electric 

 machine is of great interest: in its commercial applica- 

 tion, it is also of great value; for as nothing but motion 

 is required to put it into action, all the expenses and 

 annoyance of liquids, <bc., used in exciting voltaic bat- 

 teries, are obviated. 



t See p. 113, el tej. ; and figi, SO, 81, 82, and 89. 



