Dec. 8. 1882. 



o KNOWLEDGE 



449 



much in the first days of returning sunshine. Some trees 

 carry this principle of provision for the future one step 

 further, and lay up their flower buds as well as their leaf 

 buds four months beforehand. For example, look at this 

 little sprig which I have just picked from the overhanging 

 alder. You see at once that it is covei-ed by two kinds of 

 buds, one of which clearly foreshadows the future foliage, 

 while the others are just as distinctly unripe catkins. At 

 the very first approach of warmer weather in earliest spring, 

 the alder catkins burst out at once into full bloom, and so 

 succeed in getting their tiny cone-like fruits well set by 

 wind-fertilisation, long before even the willows and the 

 elms have been duly impregnated by the honey-seeking 

 bees. 



THE AMATEUR ELECTRICIAN.* 



ELECTRICAL MEASUREMENT.— VI. 



HAVLNG described the more important units adopted 

 in electrical measurements, it behoves us next to 

 direct our attention to the apparatus employed in making 

 such measurements. Of all the instruments used for this 

 purpose, none are so generally adojited as the galvanometer. 

 The ordinary form, consisting of a coil of wire surrounding 

 a magnetised needle, which is deflected by the passage of a 

 current through the coil, may, however, be more fitly 

 described as a galvanoscope. 



But " to obtain accurate results, we require accurate 

 observations, made on accurate instruments, by accurate 

 methods." The tangent galvanometer is an instrument 

 which, if properly liandled, is capable of recording most 

 accurately, and as its construction is somewhat easy, a 

 detailed description of it will doubtless be of some 

 service. The illustration (for the use of which we are 

 indebted to the courtesy of the proprietors of the 

 Electrical Review), represents, with one or two little 

 omissions, the tangent galvanometer used in the tele- 

 graph service. It is made chiefly of brass, but good 

 drij wood, although, perhaps, less elegant, will answer 

 the purpose quite as etiiciently. In the liase plate or 

 block, Gi in. in diameter, are four brass terminals or 

 binding-screws, and supported by a short stout pedestal 

 is a flat ring, li in. wide, with an external diameter of 

 S in. It is about ^ in. thick, with a deep groove \ in. wide 

 cut on the outer edge. In this groove several layers of 

 No. 35 copper wire are wound, enough to ofler a resistance 

 of aboutioO Ohms, the endsof the wire being attached to two 

 of the above-mentioned binding-screws. Outside this fine 

 wire a few layers of Xo. IG wire are wound and connected 

 to the other pair of binding-screws. Care must be taken 

 that the insulating material is not rubbed ofli" the wire, and 

 it would be advisable to fill up the groove with some insulat- 

 ing substance. Where elegance or neatness is desired, it 

 would be best to lead the w ires, through the pedestal, to the 

 base, and make the connection underneath. The needle-case 

 is fixed across the centre of the vertical ring, and consists of 

 a box about an inch deep inside, with a glass cover through 

 which the needle can be seen. The needle is rather diflicult 

 for an amateur to make, and to secure efficiency it had 

 better be placed in the hands of a competent workman. 

 Essentially it consists of a small magnet about J in. 

 long, with a tine brass wire, or pointer, about 4| in. long 



In " Measurement V.," on page 103, read 

 ■08 for 04 ; anil on page lOi resul . .— .= G 00 

 T = G ix! =1-212. 



•1 for 



attached to it at right angles, an agate pellet being fitted 

 in a cap provided for it. The whole revolves on a fine 

 steel point. The base of the box is graduated into degrees 

 of a circle on one side of the ring, and into divisions pro- 

 portional to the tangents of the angles on the other side. 

 A stout brass wire fixed to the upper edge of the ring 

 carries a sliding tube with a slightly curved " controlling " 

 magnet. The tube fits " spring-tight," to prevent it 

 falling from any portion of the wire to which it may be 

 moved, the function of the magnet being to increase or 

 decrease the sensitiveness of the needle in proportion as it 

 is raised or lowered. 



The little " key " seen between the binding screws on tlie 

 right of the figure is used for short-circuiting the galvano- 

 meter, but it may be ignored by the amateur. Its use is 

 to bring the needle to rest speedily. 1 n setting the instru- 

 ment to take a reading, it should l)e turned round until the 

 pointer is over the zero of the scale, and at right angles 

 with the vertical ring. The magnet and the ring will then 

 be in the magnetic meridian. The principle upon which 

 tlie instrument is constructed is explained in any good text- 



book. 



lay. 



how- 



say that the needle being 



very short in comparison to the diameter of tlie ring, tlie 



