ADDHK-Sy. 



in educating those so-called practical electricians whose ideas do not 

 easily rise above ohms and volts. It has long been known <hat when 

 the changes are sufticiently rapid, the phenomena are governed much 

 more by induction, or electric inertia, than by mere resistance. On this 

 principle much may be explained that would otherwise seem paradoxical. 

 To take a comparatively simple case, conceive an electro-magnet wound 

 with two contiguous wires, upon which acts a given rapidly ])eriodio 

 electro -motive force. If one wire only be used, a certain amount of heat 

 is developed in the circuit. Suppose now that the second wire is brought 

 into operation in parallel — a proceeding equivalent to doubling the section 

 of the original wire. An electrician accustomed only to constant currents 

 would be sure to think that the heating effect would be doubled by the 

 change, as much 1 eat being developed in each wire separately as was 

 at first in the single wire. IJnt such a fonclusion would be entirely 

 erroneous. The total current, being governed practically by the self- 

 induction of the circuit, would not bo augmented by the accession of the 

 second wire, and the total heating effect, so far from being doubled, would, 

 in virtue of the superior conductivity, bo halved. 



During the last few years much interest has been felt in the reduction 

 to an absolute standard of measurements of electro-motive force, current, 

 resistance, etc., and to this end many laborious investigations have been 

 undertaken. The subject is one that has engaged a good deal of my 

 own attention, and I should naturally have felt inclined to dilate upon 

 it, but that I feel it to be too abstruse and special to be dealt with 

 in detail upon an occasion like the present. As regards resistance, I will 

 merely remind you that the recent determinations have shown a so greatly 

 improved agreement, that the Conference of Electricians assembled at 

 Paris, in May, have felt themselves justiGed in defining the ohm for 

 practical use as the resistance of a column of mercury of 0° C, one square 

 millimetre in section, and 106 centimetres in length — a definition differing 

 by a little more than one per cent, from that arrived at twenty years ago 

 by a committee of this Association. 



A standard of resistance once determined upon can be embodied in 

 a ' resistance coil,' and copied without much trouble, and with great 

 accuracy. But in order to complete the electrical system, a second standard 

 of some kind is necessary, and this is not so easily embodied in a permanent 

 form. It mifUt conveniently consist of a standard galvanic cell, capable 

 of being prepined in a definite manner, whose eloctro-motive force is once 

 for all determined. Unfortunately, most of the batteries in ordinary use 

 are for one reason or another unsuitable for this purpose, but the cell in- 

 troduced by Mr. Latimer Clark, in which the metals are zinc in contact 

 with saturated zinc sulphate and pure mercury in contact with raercurous 

 sulphate, appears to give satisfactory results. According to my measure- 

 ments, the electro-motive force of this cell is 1'435 theoretical volts. 



We may also conveniently exjiress the second absolute electrical 



