INTRODUCTORY REMARKS.] UNDUL ATOR Y FORC ES. E LECT RO-M AGNET ISM. 247 



CHAPTER II. 

 ELECTRO-MAGNETISM. 





No branch of experimental science has made such rapid 

 progress as tliat to wliich we shall now direct attention. 

 This is chiefly owing to the discovery of voltaic elec- 

 tricity, by means of which we have the power of obtain- 

 ing any amount of current force of great quantity, which 

 is an essential point in all electro-magnetic researches. 

 For most of the experiments which we shall adduce, but 

 little apparatus is required, and such can easily be con- 

 structed by attention to the directions we shall give. A 

 voltaic battery of either Daniell's, Smee's, or Grove's 

 form, will bo required ; one cell, however, being suffi- 

 rii-nt for most purposes. When larger arrangements are 

 needed, we shall mention the fact as we proceed. A 

 quantity of copper wire, covered with cotton or silk, of 

 No. 10 and 22 gauge ; a strong horse-shoe magnet, and 

 some good steel needles, will also be requisite for the 

 majority of the experiments. With these the student 

 ike an extended investigation of the leading laws 

 and phenomena of electro-magnetism, and thus acquire 

 a competent knowledge of the subject at a trifling 

 expense. 



In studying the facts of which we shall have to 

 speak, several difficulties may arise in the minds of 

 those of our readers who are not prepared to take an 

 Independent view of the action of two forces, which, 

 whilst Mtmi&fi to be antagonistic, are really due to the 

 same cause. For instance, wo shall show that a current 

 of electricity which produces magnetic phenomena, in 

 some cases acts in opposition to the force it may gene- 

 rate. This difficulty, however, if carefully examined, 

 will prove rather an advantage than otherwise to a 

 careful and attentive student, because it will call out 

 his reasoning and observing powers, and so add, as far 

 as he is concerned, an additional interest and value to 

 this branch of philosophical pursuit. We must, how- 

 ever, remark, that all such opposition of the forces to 

 which we refer, is more apparent than real, ami results 

 from the direction of the electro-current being-modified 

 in reference to the plane of the magnetic force : this, 

 however, we shall fully inquire into as we proceed. 



Perhaps the most convenient and instructive course 

 which we can take in introducing our readers to the 

 laws of electro-magnetism, is that of proving, in the first 

 place, that a current of voltaic electricity affects the 

 magnetic needle when certain particulars are observed in 

 carrying out the experiments which illustrate this fact. 

 This may readily be ascertained by means of the follow- 

 ing arrangement, which we suggest as one of the simplest 

 which can be used for the purpose : 



Experiment 1. Fill a basin nearly to its brim with 

 water, and float on its surface a magnetised sewing- 

 needle.* When the needle has ceased to vibrato, and is 

 therefore quite at rest, place over, and exactly parallel 

 to it, a straight copper wire, at a distance of about a 

 quarter of an inch from the needle, so that the two shall 

 lie on the same straight line. Now, so long as no cur- 

 rent of electricity passes by the copper wire, the needle 

 will not be deflected from its position ; but if a current 

 of electricity be sent from one cell of any form of the 

 voltaic battery, then the needle will instantly be 

 moved from its position, and be deflected right or" left, 

 according to the direction of the current. A voltaic 

 battery, for this experiment, is easily made by soldering 

 a wire to a small piece each of zinc and copper. These 

 metals are then to bo placed in a vessel containing 

 water, to which a tenth part of its bulk of sulphuric 

 acid has been added. On the terminal wires being 

 brought in contact with the wire placed over the needle, 

 the deflection of the latter will be immediately ob- 

 served. Of course the two plates of metal must not bo 

 See ante, p. 144. 



allowed to touch each other. The following engraving 

 illustrates the arrangement we have described, and 



fig. 3. 

 a 



shows the deflection of the needle during the passage 

 of the current by the wire. 



fixjKriment 2. We now shall point out how the 

 deflection takes place in reference to the direction of 

 the current by the wire placed over it ; and for this 

 purpose we shall consider that that end of the needle 

 nearest a, in Fig. 3, is its north pole, and that near 6 

 to be its south pole. Bring the wire from the copper or 

 platina plate of the battery cell into contact with the 

 wire over the needle at a (Fig. 3), and that from 

 the zinc end of the cell into contact with the wire at b. 

 The current of electricity passes tit the battery from the 

 zinc to the copper, and thence by the wire from a to 6. 

 In this direction it will deflect the needle towards the 

 right hand of the observer, supposing that he looks at 

 the needle from its south, towards its north pole. After- 

 wards, remove the wires of the battery, and adjust the 

 needle so that it may again bo at rest, parallel to the 

 wire passing over it on the basin. Then allow the cur- 

 rent from the battery to pass from 6 to a by placing the 

 wire from the copper plate at 6, and that from the zinc 

 plate at a. The needle will now be deflected towards 

 the observer's left liaud, the current passing from 6 to o. 



This will be still further Fig. 4. 



illustrated by means of o 



the annexed diagram ; in 

 which No. 1, Fig. 4, re- 

 presents the current as 

 passing downwards from 

 a to 6, in the direction 

 of the arrows ; whilst, in 

 No. 2, it is passing in the 

 direction from 6 to a. The 

 north pole of the needle is 

 thus diverged to the right 

 in No. 1, and to the left in 

 No. 2, owing to the reversal 

 of the current in the latter 

 case. It ia of the greatest importance that tlie student 

 should bear in mind these laws of deflection, as they 

 exercise groat influence in all the phenomena of electi'o- 

 magnetism ; and, although our engravings are explicit 

 enough to explain the facts, it is far better that the 

 experiment should bo frequently repeated in the simple 

 manner we have just described, as that will certainly 

 impress tho truth more firmly on the memory than 

 any amount of study could do. We may hero add, that 

 when we describe the form of electric telegraphs in 

 common use in this country, we shall presume that the 

 facts wo have thus illustrated have been fully under- 

 stood by our readers. 



From these experiments we can readily deduce tho 

 first law of electro-magnetism; which is, "That a cur- 

 rent of electricity affects tho magnetic force at right 

 angles to the direction of the current itself." In apply- 

 ing the law practically to tho two last experiments, wo 

 observe, that as the current passes by the wire, the 

 magnetic needle assumes a direction approaching that 



