238 



• KNOVvALEDGE ♦ 



[April 20, 1883. 



THE AMATEUR ELECTRICIAN. 



ELECTRICAL MEASUREMENT.— X. 



IN order to ol>t,ain a conveniontly-conipU^to set of Bridge 

 coils, it is ndvi?al>le to construct a small resistance- 

 box, holding four coils, so adjustable as to be able to 

 perform the functions of Coils A and B (Fig. 2, Electric-1 

 lleasurenient VIII.). 



V 



The accompanying diagram (Fig. 1) illustrates a simple 

 form, based on a design similar to that previously described 

 (Electrical Measurement VII.). M is a sheet of ebonite or 

 other insulating material, about a quarter of an inch thick, 

 and S inches long, by 2^ or 3 inches wide, and forming the top 

 of a box an inch and a quarter to an inch and a half deep. 

 Four ebonite, ebony, or boxwood bobl<ins are screwed to 

 the under side of M as shown at T R S U, the bobbins 

 being first carefully wound with silk-covered wire by the 

 " double-winding " method (Electrical Measurement VII. ), 

 two of them, E, and S, offering ten ohms resistance, 

 and the other two, T and U, otiering 100 ohms each. 

 The coils T and U are connected by one of their extremities 

 to the under screws of Y and Z respectively, the other 

 extremities being similarly connected to W and X. In 

 the same way, R and S are connected at one end 

 to \V and X respectively, the other ends of the 

 10-ohm coils being both connected to the central bind- 

 ing-screw, V. Short straps (Fig. 2, Electrical Measure- 

 ment VII.) are placed between the binding-screws on the 

 upper side of the ebonite plate, and removed or discon- 

 nected at one end when the subjacent resistances are 

 required. An extra nut is provided on the central and on 

 each of the end binding-screws (V Y Z) for the necessarj' 

 wire connections. 



One pole of the battery being put to earth, the other 

 pole is joined to V, where the current divides, one portion 

 going to Y and the other portion to Z. The resistance- 

 box previously described (Electrical Measurement, VII.), 

 is connected by a short piece of copper wire to Y (or Z), 

 and the wire, or whatever it is the resistance of which is 

 desired to be measured, is connected to Z (or Y). The 

 remote ends of the resistance coils and of the wire being 

 both either put to earth or brought back to the battery 

 and connected to the pole which would otherwise be put to 

 earth. 



The galvanometer should be connected to Y and Z. It 

 should, to be efficient, be wound with several layers of 

 fine silk-covered wire, and the critical parts very carefully 

 constructed. Too much stress cannot, in fact, be laid upon 

 the necessity for the greatest care to be exercised in the 

 construction of this instrument. The magnet should be 

 comparatively small, and entirely enveloped by the coil, a 

 light indicator being attached to it at right angles, and 

 made to travel over a horizontal scale with the zero accu- 

 rately marked. It is a great advantage to have a piece of 

 silvered glass under it, so that, in looking down on the 

 indicator, the shadow would be invisible, and a correct 

 reading taken. If properly made the galvanometer should 

 indicate, with low resistances and a powerful battery in 

 circuit, an error in measurement of a hundredth of an ohm. 



The cost of such a box as the one illustrated would be very 

 small, while the arrangement of the coils and termiaaU 

 render the apparatus of great utility in a variety of ways. 

 It may be used as an ordinary set of resistance coils, 

 and as a Bridge coil it permits of a very useful series of 

 proportions. Suppose, for example, we wish to measure 

 the resistance of a length of wire, that we are not par- 

 ticular to the fraction of an ohm, and that we have reason 

 to believe it will give, say between 200 and 300 ohms. Then, 

 disconnecting the straps over T and U, we vary the resist- 

 ance in the larger box until the needle is unaffected, and 

 the resistance of T and U (representing A and B in the pro- 

 portion mentioned near the end of Electrical Measurement 

 JX.) being equal, the resistance of the wire being tested is 

 equal to that in circuit in the large box. 



Suppose, in the next experiment, that we believe the 

 wire which we join to Z ofiers between 2,000 and 3,000 

 ohms. Then, after short-circuiting T and disconnecting the 

 strap over R (S being short-circuited), we again vary the 

 resistance in the large box connected to Y, until equili- 

 brium is produced. If this is obtained when 250 ohms are 

 inserted between Y and earth, it is evident that the re- 

 sistance of the wire being tested is 2,500 ohms, as shown 

 by the simple proportion : — 



10 : 100 :: 250 : 2,500 



As a third experiment let us assume that our wire is 

 under one ohm, then we should short-circuit U and R, and 

 disconnect the straps over S and T, when, if the needle be- 

 come stationary with five ohms in the large box, the wire 

 will offer halt an ohm, thus : — 



100 : 10 :: 5 : -5 



If, under the same conditions, a balance is effected with 

 2-5 ohms, then the resistance of the wire will be a quarter 

 of an ohm, thus : — 



100 : 10 :: 2-5 : -25 



It is, generally speaking, advisable to have an arrange- 

 ment for easily disconnecting and reconnecting the battery, 

 otherwise there will, under certain circumstances, be great 

 risk of injuring tlie galvanometer. Fig. 2 illustrates, in 

 elevation, a simple device. AB is a block of ebony or 

 other hard wood, two or three inches square, and half an 

 inch or so thick, c D is a piece of thin brass or steel, 

 about half an inch wide, rigidly fixed at one end by the 

 screw E, and having just enough spring in it to separate G 

 from F, when the finger, which by pressing the ebony 

 knob 11 lirouglit G into contact with F, is removed. F is 

 a small contact stud, consisting, preferably, of a piece of 

 platinum soldered to a brass or iron screw. It may, how- 

 ever, consist simply of a wood screw, screwed in from the 

 underside of A B, the head being well sunk, so as not to 

 protrude below the under surface. Similarly, G is a small 

 piece of platinum wire or foil soldered to c D, but may, if 

 desired, be dispensed with. The reason assigned for using 

 platinum is, that it is much less oxidisable than brass or 

 iron, and therefore preserves a clean contact surface. This 

 " key " is placed between the battery and V of the Bridge 

 coils, one wire being connected to E and the other to a 

 binding screw, placed on any convenient part of A B, and 

 connected to F. By this means, when II is depressed, the 



