238 



NATURE 4a . f-^^S" lJ^-ly9, 1891 



measured by the aid of a voltmeter V across its terminals. This 

 obviates the necessity of putting an electro -magnetic instrument 



in what should be a non-inductive circuit. The formula for the 

 mean watts spent in the circuit ab. Figs, i and 2, are re- 

 spectively — 



W 



= —N\ - VJ - V|\ and W ^ ~iK.\ - AJ 



- A?.l 



Mr. Blakesley's method of measuring power by a split-dynamo- 

 meter was shown to be analogous to the original electrometer 

 method in which the difference of two readings was proportional 

 to the power, and Blondlot and Currie's double electrometer 

 method was shown to be the analogue of the ordinary wattmeter. 

 The wattmeter was defective in the fact that a solenoidal coil was 

 inserted in a nominally non-inductive circuit. The error thus 

 introduced is, as was shown by one of the authors some years 

 ago, expressed by the formula — 



Apparen t watts _ I -f tan . tan d) , 

 True watts i -t- tan^ <> 



where Q is the phase an^le between the current and E.M. F. in 

 the circuit in which the power is to be measured, and <^ the 

 phase angle for the approximately non-inductive circuit. It is 

 now proved that the same formula expresses the error in any of 

 the methods where resistances not wholly non-inductive are used. 

 As is well known, Mr. Blakesley has applied his split-dynamo- 

 meter to the measurement of phase differences between two 

 currents ; and an analogous method of finding the phase difference 

 between two potential differences is described in the paper. In 

 this method a high resistance split-dynamometer such as suggested 

 by Mr. Rimington for measuring power is employed. Blondlot 

 and Currie's double electrometer could also be used for the 

 same purpose. Numerous diagrams illustrating the various 

 analogies accompany the paper. Prof. S. P. Thompson inquired 

 whether hot-wire voltmeters could be employed to measure the 

 various potential differences, without introducing error. In reply, 

 Prof. Ayrton said that, although no great error was introduced 

 by the self-induction of these instruments, yet the fact that they 

 required considerable current was a disadvantage, and as these 

 currents were not always in the same phase as those in other 

 circuits, troublesome corrections were sometimes necessary. 

 Electrostatic instruments were preferable. Prof. Adams said hewas 

 glad to hear that the inductance of Cardew voltmeters introduced 

 no serious error, for they were very convenient instruments to 

 use. — Prof O. Lodge, F.R.S., exhibited and described a clock 

 for pointing out the direction of the earth's orbital motion in the 

 ether. After mentioning the various motions to which a point 

 on the earth's surface is subjected, he pointed out that the orbital 

 motion was the largest component, and its direction at any in- 

 stant not easy to conceive. An apparatus for pointing out this 

 direction was therefore convenient when dealing with problems 



NO. II 3 2, VOL. 44] 



requiring a knowledge of the motion of a point through the 

 ether. In one of two clocks shown, one spindle representing the 

 earth's polar axis and another the axis of the ecliptic were in- 

 clined at an angle of 23!", and coupled by a Hooke's joint. The 

 latter axis was capable of rotating round the former. At its upper 

 end the ecliptic axis carried a tube and a pointer, both being 

 perpendicular to the axis and to each other. The clock keeping 

 solar time rotated both axes, and when properly set the tube 

 pointed in the direction of the sun, and the pointer LheiefQtfi-io.- 

 dicated the direction of the earth's orbital n,)aliun;^Someex- 

 periments with Leyden jars were then shown by^^Dr. Lodge. 

 The first one was with resonant jars, in which the discharge of 

 one jar precipitated the overflow of another, when the lengths of 

 j the jar circuits were properly adjusted or tuned. The latter jar 

 I was entirely disconnected from the former, and was influenced 

 j merely by electro-magnetic waves emanating from the discharging 

 ' circuit. Lengthening or shortening either circuit prevented the 

 \ overflow. Correct tuning was, he said, of great importance in 

 I these experiments, for a dozen or more oscillations occurred 

 [ before the discharge ceased. The effect could be shown over 

 [ considerable distances. In connection with this subject Mr. 

 J Blakesley had called his attention to an observation n.ade by 

 I Priestley many years ago, who noticed that, when several jars were 

 being charged from the same prime conductor, if one of them 

 I discharged the others would sometimes also discharge, although 

 I they were not fully charged. This he (Dr. Lodge) thought 

 might be due to the same kind of influence which he had just 

 shown to exist. The word resonance, he said, was often mis- 

 I understood by supposing it always had reference to sound, and 

 1 as a substitute he thought that symphoning or symphonic might 

 I be allowable. The next experiment wes to show that wires 

 might be tuned tO' respond to the oscillation of a jar discharge 

 just as a string could be tuned to respond to a tuning-fork. A 

 thin stretched wire was connected to the knob of a jar and 

 another parallel one to its outer coating, and by varying the 

 length of an independent discharging circuit, a glow was caused 

 to appear along the remote halves of the stretched wires at each 

 discharge. Each of the wires thus acted like a stopped organ- 

 pipe, the remote ends being the nodes at which the variations of 

 pressure were greatest. By using long wires he had observed a 

 glow on portions of them with the intermediate parts dark ; this 

 corresponded with the first harmonic, and by measuring the 

 distance between two nodes he had determined the wave-length 

 of the oscillations. The length so found did not agree very 

 closely with the calculated length, and the discrepancy he 

 thought due to the specific inductive capacity of the glass not 

 being the same for such rapidly alternating pressures as for 

 steady ones. He also showed that the electric pulses passing 

 along a wire could be caused (by tuning) to react on the jar to 

 which it was connected, and cause it to overflow even when the 

 distance from the outside to the inside coating was about 8 

 inches. During this experiment he pointed out that the noise of 

 the spark was greatly reduced by increasing the length of the 

 discharging circuit. The same fact was also illustrated l)y causing 

 two jars to discharge into each other, spark gaps being put both 

 between their inner and outer coatings so as to obtain "A" 

 sparks and "B" sparks. By putting on a long "alternative 

 path " as a shunt to the B spark gap and increasing that gap, the 

 noise of the A spark was greatly reduced. He had reason to 

 believe that the B spark was a quarter phase behind the A spark, 

 but the experimental proof had not been completed. He next 

 described some experiments on the screening of electro-magnetic 

 radiation, in which a Hertz resonator was surrounded by different 

 materials. He had found no trace of opacity in insulators, but 

 the thinnest film of nietal procurable completely screened the 

 resonator. Cardboard rubbed with plumbago also acted like a 

 nearly perfect screen. In connection with resonators, he ex- 

 hibited what he called a graduated electric eye or an electric harp — 

 made by his assistant, Mr. Robinson — in which strips of tin foil 

 of different lengths are attached to a glass plate, and have spark 

 gaps at each end which separate them from other pieces of 

 foil. One or other of the strips would respond according to 

 the frequency of the electro-magnetic radiation falling upon 

 it. Mr. Blakesley asked whether the pitch of the resonant 

 jars altered when the distance between their circuits was 

 varied, for according to theory the mutual induction should 

 diminish the self-induction, and cause the oscillations to be more 

 rapid. If this occurred, the method might be used for getting 

 rapid oscillations. He also inquired whether the glow would 

 appear in the same position on the two stretched wires if their 



