TKANSACTIONS OF SECTION A. 513 



being visible above a dial, and capable of a motion of nearly 360°. When a current 

 passes the pointer rotates, because of the heating of the strip. By continuous 

 making and breaking of a large current through the strip during 72 hours all zero 

 and other errors seem to be eliminated. The highest reading of the exhibited 

 instrument was 2J volts, so that it is particularly useful in accumulator work. 



The author gave the law of transformers generally, and found that a small 

 transformer on the base of the instrument converted it into a voltmeter of any 

 range whatsoever, for alternating currents, the readings being independent of the 

 frequency of alternation of the current. The instrument gave accurate readings 

 whether the strip was vertical or horizontal. The author exhibited twisted strips 

 of carbon made by Messrs. Woodhouse & Rawson, which he intends to use for 

 voltmeters of higher range for continuous current work. 



13. On the Relative Effects of Steady and Alternate Currents on different 

 Conductors. By William Henry Preece, F.B.S. 



Sir William Thomson, at the Bath meeting of the British Association, startled 

 the electrical world by asserting that alternate currents entered a distance of only 

 about three millimetres into the heart of a thick, round, copper conductor when 

 the frequency was 150. This 'Dill'usion Law,' as he called it, is based on the 

 assumption that a current starts at the surface of the conductor and works its way 

 radially inwards. It is dependent on the coefficient of self-induction and on the 

 frequency — that is, on the number of complete alternations of positive and negative 

 currents transmitted per second. 



As this law has a most important bearing on the commercial value of systems 

 of distribution dependent on alternate currents, it becomes most desirable to study 

 the question practically. The verification of the law is almost beyond the reach of 

 experiment. It occurred to the author, however, that if conductors of different 

 materials were taken, such as iron, copper, lead, and platinoid, of easily measurable 

 lengths and of convenient sectional areas, and that measurable and variable currents 

 ■were transmitted through them, both alternate and direct, approximately similar 

 to those used in practice, it would be possible to study the question by observing the 

 changes of temperature in the conductors, and one would at least observe any dif- 

 ference, if such existed, in the total expenditure of energy in the conductors under 

 these different circumstances. Any addition or diminution of energy expended 

 would give some indication of the operation of the law of diffusion, in comparatively 

 small conductors, without, however, determining the actual distribution of current 

 density. 



In the preliminary experiments that were made to determine the mode of 

 procedure and the final form of the apparatus, conductors of copper, iron, lead, 

 and platinoid were used, but in the final experiments only iron and copper were 

 employed. Each metal was taken of the same length and of the following forms : — 

 round rod, flat rod, tube, and stranded cable. The various dimensions are given 

 in the tables. The temperature was in all cases determined from the elongation of 

 the conductor. 



The observations were taken sometimes with steady currents and sometimes 

 with alternating cuiTents first. Each kind of current, after being switched on, 

 was kept constant until all expansion ceased, after which the current was instantly 

 changed to the reverse kind, and any change of elongation observed. Each ex- 

 periment lasted from twenty minutes to half an hour. The results are tabulated, 

 and some of the experiments plotted as curves on a diagram. For this latter 

 purpose observations were taken and recorded at every minute. The abscissa is 

 time in minutes, and the ordinates elongation in thousandths of an inch. 



As regards copper, it is very clear that, whether the current were steady or 

 alternating, the effect of heat generated in the conductors was virtually the same ; 

 but of the different forms the tube gave the best results, owing, doubtless, to the 

 increased radiating surface. 



The porforiuance of the iron conductor was very decided. Here there is a 

 large increment of heat, due clearly to reversals of cii'cular magnetisation. 

 1889. L L 



