VOLTAIC El. 



VOLTAIC ELECTRICITY . V. 



CNIT8 Of FORC3 AMD RESISTANCE B. A. UNIT RHEOETAT 

 BflTKCTil OF KLXCTRIO CUBKEMT ELECTRIC LJOHT 

 ELECTHIC LAMP. 



IN speaking of resistance coils in our but lesson, we stated 

 that tho iMiiU wore made of some badly-conducting wire, usually 

 <Jerman silver, and caused a resistance of a (riven number of 

 unit*. We must, therefore, now try to understand what in 

 meant by a unit of resistance. It will be easily understood 

 that if we are to compare different resistances, as has < 

 be done, or aro to ascertain the relative intensity of different 

 ti, it is necessary to have some standard, just in the same 

 way as we have standard units of time, of length, and of 

 weight. 



Many different units have accordingly been proposed. It 

 WM suggested by Whcatstone to employ a fixed length of pure 

 copper wire of a certain diameter and weight, but experience 

 bowed that sometimes this underwent considerable changes in 

 its conducting power with- 

 at tiny apparent cause, 

 owing probably to some 

 alteration in its internal 

 ,-tnirtmv, and hence this 

 : not bo relied on as a 

 .nut. Another unit, which 

 proposed by Siemens, 

 was a column of pure mer- 

 one metro in length 

 a square millimetre in 

 wction, reduced to the tem- 

 perature of 32 Fahrenheit. 

 The mercury is usually dis- 

 tilled two or three times to 

 ensure its absolute purity ; 

 the temperature, too, has to 

 be carefully adjusted, as its 

 conducting power varies with 

 every alteration in it. The 

 metre is, it should be re- 

 membered, 39-37 inches, and 

 the millimetre y^ of this 

 amount, or 0'03937 of an 

 inch. 



Several other arbitrary 

 standards, consisting of va- 

 ous lengths of wire com- 

 of different metals, 

 ave also been proposed. 

 Great inconvenience was, 

 owever, caused by this mul- 

 tiplicity of units, and accord- 

 ingly tho British Associa- 

 tion of 1864 agreed upon an 

 ideal standard, based upon 



the theoretical principles deduced from Ohm's law, and stan- 

 dard coils offering these stated resistances are now issued from 

 Kew Observatory. The unit then adopted is known as tha 

 British Association unit, or, briefly, the B. A. unit ; or some- 

 times it ia called an ohmad, after Professor Ohm. 



It would be impracticable in these lessons to explain fully 

 the way in which this unit is arrived at ; we can, however, give 

 A faint outline of the reasoning, and must refer the student who 

 Irishes to inquire more deeply into the matter to the reports of 

 the British Association. A unit force is one which generates in 

 a mass of one gramme (15'43 grains) a velocity of one metre a 

 second ; a unit pole is one which repels a similar polo at a 

 distance of one metre with a unit of force; and a unit current 

 is one which, in a wire a metre long, bent into an aro having the 

 eame radius, repels a unit pole situated at the centre of the aro 

 with a unit of force. Now the electro-motive force capable of 

 overcoming a unit of force through one metre a second Is very 

 minute, and accordingly tho British Association fixed on a 

 standard velocity of 10,000,000 metres per second, and called 

 that a unit of electro-motive force which was capable of over- 

 coming a unit of force through 10,000,000 metres per second, 

 of generating a velocity of that amount. 

 Further, Ohm's law teaches us that a unit current is pro- 



119 X.E. 



by a nit of eleetro-motive fora* in a otrotit of 

 rd*tane*. and tins, sine* we know the vats* of a nit 

 and a unit of Uotm tjotsv* force, we oan *aJe*Jsit 

 material expraseion to UM unit of nsiilsiiii The 

 by which this u don* require* the otaoet rtfi*t*a*tjt of ecpecv 

 menial and mathematical skillj the ta*k bat, however, bet* 

 accomplished. 



We give here the comparative value* of the eeveral wits, at) 

 at to enable them to be compared 



B. A. Unit ...... 



Uait 



Varlejr's Unit (a mil* copper wire, Xo. M) 

 In many investigations it u important to 



*1. 



with a greater degree of accuracy than can be done by -rrnii 

 of the resistance coil, and for this pnrpot* a simple bat highly 

 ingenious piece of apparatus, known M the Rheostat, M en- 

 ployed. This apparatus, which if the invention of Mr. Wheat- 

 stone, is represented in Fig. 29. It oooeiet* of two paraUol 

 cylinders, A and B, the former being made of braes, and per. 



fectly smooth, white is 

 mad* of dry wood, and has 

 a cloee spiral groove round 

 it from end to end. It the 

 end of B, shown ia the 

 figure, is a metallic ring 

 preeeed against the spring 

 a; to this ring i. fastened 

 one end of a fine braes wire, 

 which is than wound along 

 in the groove, and patae* 

 ultimately to the cylinder A. 

 its other extremity being 

 fattened at the end of tola. 

 A second spring praises 

 against the front end of A, 

 and to these two spring! 

 are fattened the binding 

 screws n and o. When this 

 instrument U placed in any 

 circuit, the electricity will 

 pass along all the wire which 

 it coiled on at the wood 

 insulate* it, but when the 

 wire paste* to the cylinder 

 A, it it no longer insulated, 

 and the current then ptetet 

 along the metal cylinder to 

 the spring and the binding 

 s :rew n, and to back to the 

 wire. 



Both the cylinder* 



ranged to that they turn oa 

 axles, and the ends of those 

 are squared to that the key 

 d will fit either of them. 



Thus, when it is desired to introduce a greater resistance, we 

 have merely to turn the cylinder B so as to wind more of the 

 wire upon it, and tho current will then have to travel along 

 this additional length of wire. If, on the other hand, we want 

 to diminish the resistance, we can easily do to by tm-ni^ the 

 other cylinder. 



Tho small rod between tho cylinder ia divided, so that by 

 noticing the point of it at which the wiro ere tee* the "mnrflfr 

 on either cylinder can be shown. At the further end, too, there 

 are hands which turn and show more accurately the length of 

 wire through which the current ia pasting. 



Suppose, now, that the resistances of two wires are to be 

 compared. The whole of the wire in the rheostat it first 

 wound on the wooden cylinder ; the instrument, together with 

 a galvanometer, ia then put in the circuit, and the reading of the 

 galvanometer carefully noted. One wire it now introduced into 

 the circuit, and, as the resistance ia of course increased by this, 

 the reading will be lower. A portion of the wire it therefor* 

 thrown out of the circuit by being wound on to the cylinder A, 

 and this ia continued till the reading it exactly the eame a* 

 before ; tho amount of wire transferred to the metal cylinder U 

 then read off, and shows the amount of resistance caused by the 

 first wiro. The other is then introduced into the circuit, and 



