May 5, 1882.] 



• KNOVVL^EDGh. 



569 



is represented by Figs. 1 and 2. Fig. 1. is a vertical sec- 

 tion (full size), while Fig. 2 is a longitudinal section a little 

 Ifss than half-size. The dimensions here given are such as 

 will produce a small instrument of great service for pur- 

 poses where only a few cells are otherwise required. Larger 

 ui.'ichines can be described subsequently. 



The armature is simply an electro magnet with the core 

 riattened out, and its extremities extended so as to form 

 pole-pieces of a segmental section. The core, C, in our 

 machine is four inches long and one inch wide, the thickness 

 lieing not more than three-eighths of an inch. The pole- 

 pieces, A and B, are ti\-e and a-half indies long, quarter of 

 ail inch thick in the thickest part, and turned up so as to 

 form parts of a circle of one and a-half inches diameter. 

 The armature may be made in three pieces, and fastened 

 together by means of 'wTOught-iron screws or rivets ; or, 

 what is better, it may Vie cast in one piece, great care 

 Ix'Lng taken to ensure that the iron is perfectly soft, and 

 possesses no coercive force — that is to say, it must not 

 retain any magnetic polarity after a current of electricity 

 has passed round it It must also be pointed out 

 that when in working order, the armature has to 

 revolve between what are called the magnetic pole- 

 pieces bored out cylindrically, the greatest effect 

 i>eing obtained when the armature revolves in a circular 

 loring only sufficiently large to allow it to move with- 

 out touching. Consequently, the more true we make 

 the curves of the pole-pieces, the more effective will the 

 apparatus become. For the Vionetit of those of our readers 

 wlio have not the facilities for turning out such work, we 

 have deposited a pattern with a very good firm of iron- 

 founders, who -will send the casting, either in the rough or 

 finished state* 



Well insulated copper wire should be very carefully 

 wound round the core, so as to fill up the hollow portions 

 of the casting, and form as nearly as possible a circle 

 with AB, as illustrated by WW in Figs 1 and 2. It is 

 recommended to use number 21 BWG, silk-covered (price 

 .'is. ."id. per pound). Both extremities of the wire may be 

 brought to one end of the armature. Before winding on 

 the wire, gun-metal caps should be fitted ready to be fixed 

 on to the armature. Fig 3 represents one of these caps 

 in vertical section. Holes should be drilled in the face of 

 each cap, and corresponding screw-threads tapped into the 

 pole-pieces of the armature, so that perfect rigidity may, on 

 screwing together, be ensured. The caps should also be 

 furnished with projections (about half-an-inch thick and 

 an inch in length), whose function is to act as a spindle. 

 A small pulley wheel, say an inch in diameter, should be 

 driven (or cast) on to one spindle projection, and a little 

 I'bonite tube over the other. In this latter cap two small 

 holes should be made near the spindle, and through them 



• Readers wiehinK for these castings, can get them by sending to 

 Mr. Benjamin Slater, Wellington Foundry, Charles-street, City- 

 road, London, E.G., for Knowlepge casting. No. 1, rough, 6d. ; or 

 tiniehed, Ss. 



the ends of Uie armature wire should be passed. What 

 becomes of them we will say hereafter. 



When working, the armature will revolve at the rate 

 of 1,.500 or so revolutions per minute, and it is therefore 

 essential that eveiything should be true. The wire has 

 naturally a tendency to lly out and will do so, unless two 

 or three small grooves arc made in tlie pole-pieces A, B, m 



[ZU 



Fig. 3. 



which a few turns of silk thread, or any equally tenacious 

 non-magnetic substance, are wound, passing round the 

 armature, and so securing the wire. It is as well, per- 

 haps, to remind our readers that they should make sure 

 that the entire armature is evenly balanced. This they 

 can easily find out. We will leave our description of the 

 other portions of the apparatus till next week. 



PHOTOGEAPHY FOR AMATEURS. 



By a. Brothers, F.R.A.S. 



THE solution required for developing the negative is 

 composed of protosuljihate of iron 1 oz., glacial acetic 

 acid 1 oz., alcohol 1 oz., and water (rain or distilled always 

 to be understood in making up solutions) 20 oz. In hot 

 weather more acid and less iron may be used. The iron 

 should be dissolved before the acid and alcohol are added. 

 Filtration is not necessary if the solution be allowed to 

 settle. 



When the plate is removed from the camera slide or 

 carrier, it presents the same appearance as when placed in 

 it (the film of iodide of silver in the collodion is creamy 

 white) ; no image whatever is visible ; it is latent, and 

 requires to be " developed." Several agents are used for 

 this purpose, pyrogallic acid and protosulphate of iron being 

 most commonly employed. Development with pyrogallic 

 acid is very slow as compared with the iron, and for 

 that amongst other reasons the iron is mostly used. The 

 light has caused a chemical change, and has so modified 

 the silver compound that when the reducing agent is 

 poured over the plate the silver is thrown down in a 

 metallic state on those parts which have been acted upon 

 by the light, in exact proportion as it has been reflected by 

 the object which has been copied. This will be seen to be 

 the case when the negative is examined by transmitted 

 light ; the lightest parts will appear most opaque, and the 

 deepest sliades .show only bare glass. That the image is 

 formed of metallic silver may be proved by gentle friction 

 on the dry surface of the negative, as, after removal of the 

 powdery surface, the polished silver will be found beneath. 

 In developing the picture, only sufficient solution 

 should be used to cover the plate, and this, of course, 



