70 



KNOWLEDGE 



[Feb. 2, 1883 



know what is meant, wliile thousands of husbuiuls will not 

 only know, but di'eply feci tht' meaning of tliis liiiit. 



[Dr. Lewis's closing words are specially addressed to his 

 own nation, but tliey may be read with advantage here.] 



One is led to say that the microscopic girls that swarm 

 about our schools and chatter in our streets are the 

 curiosities of what we call "high civilisation." They are 

 found only among the lacing peoples. Wherever women 

 give free play to their lungs and stomachs, they grow as 

 large, or nearly as large, as men. Tliis " high civilisation " 

 is curious. Its avowed aim is a nobler manhood and 

 womanhood. But while we are so proud of our telegraphs 

 and railroads, and grand invention.s, and magnificent im- 

 provements, and large corn-crops, that we run our printing- 

 presses all night to proclaim our glory to the rising sun, 

 our doctors, standing in the midst of a nation of men suck- 

 ing tobacco, caution a nation of corseted women to go slow 

 and lie fiat on their backs three months every year. 



THE AMATEUR ELECTRICIAN. 



ELECTRICAL MEASUREMENT.— A'll. 



IN order to make the most of the tangent Galvanometer, 

 a few resistance " blocks " or coils will be necessary. 

 The wire most serviceable for this purpose will depend upon 

 the current intended to be sent through it. When only a 

 small resistance is required, it is in all cases best to use 

 copper wire, but, for high resistances, German silver should 

 be used, as the amount of copper required in such cases 

 would be cumbersome. German silver compared to copper 

 of the same length and sectional area, and at the same 

 temperature, ofl'ering a relative resistance of about 13 to 1, 

 considerable resistance can be introduced into a circuit by 

 the use of thin wire of that metal. A " resistance-box " 

 consists of a series of coils of wire offering various resist- 

 ances, which can be inserted in or withdrawn from a circuit 

 at pleasure, and with little or no trouble. 



Resistance - boxes, or, as they are sometimes called, 

 " Rheostats," are made in various forms, all more 

 or less simple in appearance. Considerable mechanical 

 skill is, however, necessary in the construction of most 

 of them, because of the necessity that contact, where 

 it is to be made, should be as perfect as the materials 



■will permit. In the plan we are about to suggest, there- 

 fore, our object is to combine mechanical simplicity 

 with electrical efficiency. A number of small bobbins 

 (B, Fig. 1) of ebonite, ebony, or boxwood, are wound 

 with wire sufficient to give their required resistances. 

 Some little difficulty may possibly be encountered in 

 measuring the resistance of the wire ; but that is a 

 branch of the subject the discussion of which we must 

 reserve till our next article.* 



In winding the coils, the wire, after being measured for 

 resistance, should V)e doubled in the centre, from which the 

 winding should be commenced. 



If wound singly or continuously, the current, in passing 

 through any given portion of the coil, induces in other 

 portions opposing currents of electricity, which frequently 

 prove very troulilesome. This induction the method of 

 double winding overcomes, as the current practically tra- 

 verses two coils in opposite directions, in which the induc- 

 tive effect is equal and opposite, neutrality being thereby 

 produced. The bobbins are then fastened in rows by screws, 

 or otherwise, on the under side of a .sheet of ebonite or 

 ebony, E E, about a quarter of an inch thick, the distance 

 between the screws, S S, being about 1 .', in. (The distance, 

 is, however, quite immaterial, and is only governed hj the 

 dimensions of the bobbins, &c.) Between the bobbins, 

 brass " Idnding-screws," C D, pass through the plate, E E 

 to which they are screwed on the under or upper surface — 

 the latter by preference. To the under portion of each 

 binding-screw one of the extremities of each of the two 

 adjacent coils is then screwed by means of the movable 

 nut, N. Pieces of stout copper wire — or, better still, brass 



Cvi 



K) 



straps shaped as shown in Fig. 2 — connect the upper por- 

 tion of each binding-screw to the similar portion of those 

 adjacent. 



0-- 



DO — I- 



EO— 10- 



HO- 



-5 — OC 



-50 — OF 



* Meanwhile, those of our readers who are unable to secure the 

 use of a "standard" resistance coil are invited to send to "The 



Fig. 3. (the figures in the last column should have 

 been -i, 4, 40, 400.) 



Fig. 3 illustrates a device for sixteen coils, ranging in 

 resistance from •! to -100 Ohms. The coils are arranged 

 in four rows under the ebonite plate, which measures 9 in. 

 by 7.1 in. In the diagram, the circles represent the ter- 

 minals or binding-screws, and the figures represent the 

 resistance of the coil of wire connected to the adjacent 

 terminals. Little paper or ivory labels may be fastened 

 over the screw-heads (S Fig. 1), indicating the resistance 

 offered by the subjacent coil. The ebonite plate Viecomes 

 the top of a shallow box — say an inch in depth. In addi- 

 tion to the straps normally connecting the adjacent binding- 

 Electrician," KNowt.F.nGE Office, 74-76, Great Queen-street, W.C., 

 short lengths (about two yards each) of about No. 35 B.W.G. copper 

 wire and Nos. 24 and 35 German silver wire (all silk-covered). A 

 stamped directed envelope being enclosed, the specimens will bo 

 measiu-ed and returned for use as standards of comparison. 



