May 2G, 1882.] 



KNOWLEDGE 



619 





THE TOTAL ECLIPSE. 



THE accounts hitherto received give very imperfect 

 ideas as to what has been done, and the telegrams 

 seem to have been despatclied with careful instructions to 

 leave entirely unnoticed whatever has been done by some 

 of the most important members of the observing force. 

 We hope next week to have letters from tlie scientific seat 

 of war. In the meantime, we note that MM. ThoUon and 

 yripied have determined the exact place of the bright 

 line in the spectrum of the corona identified with 

 1474 of Kirchholl''s scale. Tacchini and Tliollon could 

 see no dark lines in the spectrum of tl»c corona, 

 nsing diflVrent dispersions, so that one or other miglit 

 be expected to have seen such lines if on this occa- 

 sion (as in the last Indian eclipse) they had been visible. 

 Lines of hydrogen, and many bright lines seen in the 

 spectrum of the corona, were studied by ThoUon. The 

 absorption lines observed in Group B (atmospheric) were 

 obser\ed by Trepied and Thollon to be stronger near the 

 moon's edge, which has been described in the telegrams as 

 " indicating a lunar atmosphere." It seems to us rather to 

 indicate the weakening of the sky spectrum there. Con- 

 sidering how many spectra of stars have been observed 

 by Huggins close to the moon's edge without change in the 

 strength of the absorption lines, it would require some- 

 thing more than the comparatively rough observations 

 possible during eclipse to indicate a lunar atmosphere, 

 whether such atmosphere exists or not. Good photo- 

 graphs were taken (we trust Mr. Ranyard met with good 

 success in this direction), and in these a fine comet close 

 by the sun is well shown. Let us hope it is not the 

 "Spectator's comet" More of this anon. 



THE AMATEUR ELECTRICIAN. 



ELECTRIC GENERATORS {Co7Umued). 



IT is probable that some of our readers, in making a 

 machine, will experience some little difficulty in getting 

 a current from it, unless the very greatest care is taken to 

 ensure the perfect insulation of the wire. It would make 

 the machine much more reliable, if Ijefore winding the wire, 

 a layer of paper thinly-coated with shellac varnish is 

 fitted and fixed on to the coil of tlie ai-mature. The wire. 



Fig. 4. 



too, might with advantage be passed slowly through a little 

 melted paraffin wax, and so be covered with an insulator of 

 the highest quality. This may be very easily accomplished. 

 Fig. 4 will illustrate the arrangement. (D) is the bobbin of 

 wire (as it is received from the manufacturers), and capable 



of revolving on a horizontal pin formed by driving a pointed 

 piece of stout wire into the side of a common box, or asy 

 suitable article. (B) is a small tin box, such as a one-pound 

 biscuit-box, containing the paralliu wax (say half-a-pound), 

 and a small reel (R) to revohe on an axle formed by fasten- 

 ing a piece of stout wire into the sides of the box. A 

 spirit-lamp or Bunsen gas-burner (L) is placed under 

 tlie tin box (B), to keep the wax in a thoroughly 

 liquid state, but not sufficiently hot to produce any con- 

 siderable amount of evaporation. The armature-coil, or 

 whatever else we may wish to cover with wire, may be 

 easily attached to a spindle, so that, by turning it round, 

 we can at the same time pull the wire from the bobbin (B) 

 under the reel (R), and by using the left hand (or the right, 

 if more convenient) as a guide, the layers of wire may bo 

 made very regular and compact* 



Those who have the necessary convenience are strongly 

 recommended to measure the resistance of the coil of 

 wire. Our armature should contain about 7 lb. of No. 24 

 wire ofl'ering a resistance of 6 9 or 7 ohms. The chief 

 danger is that of the wire coming into contact with 

 the iron. This is a fault which may be easily detected 

 by using a battery cell with a galvanometer, such as the 

 one described in our firet article, in circuit. Take the two 

 wires (one from the galvanometer, and the other from one 

 pole of the battery) ; connect one to one end of the arma- 

 ture-coil, and hold the other on to the iron itself. If a 

 deflection is obtained, it is clear tliat a current is passing — 

 that is to say, the wire is bared somewhere, and is touching 

 or making contact with the iron. 



Before considering the armature as finished, we must 

 decide on and arrange our " commutator," or current 

 reverser. On to the plain gun-metal cap drive a boxwood or 

 ebony ringt or cylinder half-an inch long, tight enough to 

 prevent it slipping, and on to this fix by small screws a 

 brass cylinder cut longitudinally into four sections, so that 

 we shall have four pieces of metal represented in section 

 by A B C D (fig. .5) ; then join one end of the coil of wire 

 to A and the other to B. 



Now for the " ineluctor." In very small machines the 

 magneto principle is preferable to what we have yet to 

 describe as the dynamical. In a purely magneto machine, 

 the currents of electricity obtained from the revolving arma- 

 ture are generated by the inducing force of permanent steel 

 magnets. These should be very strong, each capable of sup- 

 porting, under the most favourable circumstances, at least 

 7 a times their own weight The Jamin magnetj (a Parisian 

 manufacture) is capable of sustaining fifteen times its 



* Another method of paraffining is to heat some of the wax in a 

 porcelain evaporating dish, and pass the wire through a loop made 

 in a piece of glass rod, dipping below the surface of the wax. 



t These substances are recommended where there is any diffi- 

 culty in procuring ebonite. 



I This is simply a compound magnet composed of a number 

 of strips of very thin steel placed one over the other, the surfaces 

 being coated with vamisli. 



