Feb. 6, 1879] 



NATURE 



lever working on an axis, armed with a counter-weight and with 

 a lever whose angular position will be materially affected by 

 any small elongation of the strip that may take place from any 

 cause. The apparatus further consists of a number of prisms of 

 metal, supported by means of metallic spr.ngs, so regulated by 

 movable weights as to insure the equidistant position of each 

 prism from its neighbour. 



A series of comparatively thin coils of wire of German silver 

 or other resisting metal, connect the alternate ends of each two 

 adjoining springs, the first and last spring being also connected 

 to the lever and terminal respectively. 



The strip is put under a glass shade, and the instrument itself 

 should be placed in a room where a tolerably uniform tempera- 

 ture of say 15° C. is maintained. 



When the minimum current is passing, the thin metallic strip 

 is at its minimum working temperature, and all the metallic 

 prisms are in contact, this being the position of least resistance. 

 As soon as the current passing through the apparatus shall in- 

 crease in amount, the thin metallic strip will immediately rise in 

 temperature, which will cause it to elongate, and will allow the 

 lever to recede from its extreme position, liberating one contact 

 piece after another. Each such liberation will call into action 

 the resistance coil connecting the spring end>, and an imme- 

 diate corresponding diminution of the current through the addi- 

 tional resistance thus thrown into the circuit. 



Suppose that the current intended to be passed through the 

 instniment is capable of maintaining the sensitive strip at a tem- 

 perature of say 60^ C, and that a sudden increase of current 

 takes place in consequence either of an augmentation of the 

 supply of electricity or of a change in the extraneous resistance 

 to be overcome, the result will be an augmentation of tempera- 

 ture, which will continue until a new equilibrium between the 

 heat supplied and that lost by radiation is effected. If the strip 

 is made of metal of high conductivity, such as copper or silver, 

 and is rolled do^vn to a thickness not exceeding 0*05 millim., its 

 capacity for heat is exceedingly small, and its sm-face being rela- 

 tively very great, the new equilibrium between the supply of 

 heat and its loss by radiation is effected almost instantaneously. 

 But, with the increase of temperature, the position of the regu- 

 lating lever is simultaneously affected, causing one or more 

 contacts to be liberated, and as many additional resistance coils 

 to be thrown into circuit : the result being that the temperature 

 of the strip varies only between very narrow limits, and that the 

 current itself is rendered very uniform, notwithstanding con- 

 siderable variation in its force, or in the resistance of the lamp, 

 or other extraneous resistance which it is intended to regulate. 



The resistance coils, by v/hich adjoining contact-springs are 

 connected, may be readily changed, so as to suit particular cases ; 

 they are made by preference of naked wire, in order to expose 

 the entire surface to the cooling action of the atmosphere. 



For feeble currents, disks of carbon are substituted for the 

 wire rheostat, the electrical resistance of carbon varying inversely 

 with the pressure to which it is subjected. A steel wire of say 

 0'3 miUim. diameter is drawn tight between the end of a bell- 

 crank lever and an adjusting screw, the pressure of the lever 

 being resisted by a pile of carbon disks placed in a vertical glass 

 tube. The current, passing through the steel wire, through the 

 -bell-crank lever, and through the carbon disks, encounters the 

 minimum resistance in the latter so long as the tension of the 

 wire is at its maximum ; whereas the least increase in tempera- 

 ture of the steel wire by the passage of the current causes a 

 decrease of pressure upon the pile of carbon disks, and ah 

 increase in their electrical resistance ; it will thus be readily seen 

 that, by means of this simple apparatus, the strength of small 

 currents may be regulated so as to vary only within certain narrow 

 limits. 



The apparatus first described may be adapted also for the 

 vieastu-enient of powerful electric currents. The variable rheo- 

 * at is in this case dispensed with, and the lever carries at its end a 

 i encil pressing with its point upon a strip of paper dra-\\-n'under it 

 m a parallel direction with the lever by means of clockwork. A 

 second fixed pencil draws a second or datum line upon the strip, 

 so adjusted that the lines drawn by the two pencils coincide when 

 no current. is passipg through the sensitive strip. The passage of a 

 current through the strip immediately causes the pencil attached 

 to the lever to move away from the datum line, and the distance 

 between the two lines represents the temperature of the strip. 

 This temperature depends, in the first place, upon the amount of 

 current passing through the strip, and, in the second place, upon 

 the loss of heat by radiation from the strip ; which two quanti- 



ties balance one another during any interval that the current 

 remains constant. 



In order to facilitate the process of determining the value of 

 the diagram produced by motion of pencil in Weber's or other 

 units of ciu"rent, it is only necessary, if the variations are not 

 excessive, to average the ordinates, and to determine their value 

 from a table prepared for that purpose. The error committed 

 in taking the average ordinate instead of the absolute ordinates, 

 when the current varies between small limits, is evidently small, 

 the variation of the ordinates above their mean value averaging 

 the variations below the same. 



The thin sensitive conductor may thus be utilised either to 

 restrict the amount of electricity flowing through a branch cir- 

 cuit, within certain narrow limits, or to produce a record of the 

 amount of current passed through a circuit in any given time. 



Physical Society, January 25. — Prof. G. C. Foster, vice- 

 president, in the chair. — Prof. E. Ray Lankester and Mr. 

 Alex. Macdonald, B.A., were elected Members. — Dr. Erck 

 exhibited a constant bichromate of potash battery. The ordi- 

 nary bichromate battery soon loses power when in use, and in 

 order to secure a powerful constant battery to drive a small 

 astronomical clock. Dr. Erck devised the modified form shown. 

 It consists of a narrow lead trough 12 inches long by 3 inches 

 wide and l inch deep, lined along both sides with two carbon 

 plates. The zinc plate 10 inches long is immersed in the solu- 

 tion to the depth of an inch midway between the two carbons. 

 A continual circulation of the bichromate solution is kept up by 

 allowing fresh solution to drop into the cell at one end, and the 

 exhausted solution to drop away by a tap at the other end. As 

 the space between the two carbons is only about half an inch 

 wide, there is merely a thin layer of solution between the 

 positive and negative poles. The internal resistance of the cell 

 is, therefore, very low, when short circuited only about \ ohm. 

 To obtain the maximum current about 8 oz. of solution per hour 

 should be applied. Dr. Erck also showed a battery formed of zinc 

 and carbon circular plates mounted on an axle which is rotated 

 by wheelwork, thus mechanically stirring the bichromate solu- 

 tion. — Dr. F. Guthrie, F.R.S., described some of the results he 

 had obtained from experiments on the vibration of metal rods o r 

 lathes fixed in a vice at one end and free to vibrate at the other. 

 The experiments were carried on by dusting sand on the rod 

 and observing the nodal lines formed by it when the rod was 

 vibrated, so as to give out notes determined by a monochord. 

 Dr. Guthrie's results show that the two final segments at the 

 free end are together equal in length to the inner segment at the 

 fixed end. It appears from these experiments that if a free lathe 

 vibrating with a node in the middle, but having an even number 

 of segments, be clamped at where there is a node, we alter its 

 conditions of \dbration. When the lathe is half free, the end 

 segment breaks up into two parts together equal to the segment 

 at the fixed end. In the case of a torsional vibration of the 

 lathe, the position of the longitudinal nodal lines depended to 

 some extent on the clamping of the lathe in the vice. Prof. 

 Foster pointed out that in a natural node the direction of the 

 tangent is varying, whereas in an artificial node it is always 

 horizontal. Prof. Unwin explained that the sand accumulated 

 at nodes because the particles, when thrown off the lathe, make 

 certain horizontal excursions which tend to move them nearer 

 the points of repose of the lathe. — Messrs. Elliot Brothers ex- 

 hibited sundry electric commutators and resistance boxes. 



Anthropological Institute, January 21. — Mr. John Evans, 

 D.C.L., F.R.S., president, in the chair. — The Director read a 

 communication from Dr. Paul Topinard on resemblances between 

 a Galtcha and a Savoyard skull. The similarity between these 

 skulls is such that the author is inclined to regard the Galtchas 

 of Eastern Turkistan and the Celts of Western Europe as 

 branches of one common stock, of which the Slavs of Eastern 

 Eiurope are also members. — M. Elie Reclus read a paper on cir- 

 cumcision, its significance, its origin, and its kindred rites. The 

 practice of this custom was traced over a large portion of the 

 inhabited globe, including Australia and South America, though 

 among the nations of antiquity the Egyptians and Jews are those 

 among whom it is best known to have prevaile . 



Photographic Society, Tanuar)i4. — James Glaisher, F.R.S., 

 in the chair.— Capt. Abney^ R.E., F.R.S., read a paper on the 

 fading of the undeveloped photographic image, and on soluble 

 bromide in emulsions ; and Col. Wortley explained a new in- 

 stantaneous shutter, designed by him. Capt. Abney, in his 

 paper, stated that one cause of the fading aro?e from impiu-e 



