March 13, 1884] 



NA TURE 



465 



difference? It is the sense of roughness and smoothness. Physio- 

 logists and anatomists have used the word " tactile " sense, to 

 designate it. I confess that this d es not convey much to my 

 mind. " Tactile " is merely " of or belonging to touch," and in 

 saying we perceive roughness and smoothne>s by a tactde sense, 

 we are where we were. We are not enlightened by being told 

 that there is a tactile sense as a department of our sense of 

 touch. But I say the thing thought of is a sense of force. We 

 cannot away with it ; it is a sense of force--, of directions of 

 forces, and of places of application of forces. If the places of 

 application of the forces are the palms of the two hands, we 

 perceive accordingly, and know that we perceive, in the muscles 

 of the arms, effects of large pressures on the palms of the hands. 

 But it the places of application are a hundred little areas on one 

 finger, we still perceive the effect as force. We distinguish 

 between a uniformly distributed force like the force of a piece of 

 smooth glass, and forces distributed over ten or a hundred little 

 areas. And this is the .-en-e of smoothness and roughness. The 

 sense of roughness is therefore a sense of forces, and of places 

 of application of forces, ju>t as the sense of forces in your t« o 

 hands stretched out is the sen-e of forces in [jlaces at a distance 

 of six feet apart. Whether the places be at a distance of six 

 feet or at a distance of one hundredth of an inch, it is the sense 

 of forces, and-iof places of application of forces, and of direc- 

 tions of forces, that we deal with in the sen'e of touch other 

 than heat. Now anatomists and physiologists have a good right 

 to distinguish between the kind of excitement of tissue in the 

 finger, and the minute nerves of the skin .and ?ub-skin of the 

 finger, by which you perceive roughness and stuoothnes-, in the 

 one case ; and of the muscles by uhich you perceive places of 

 application very distant, in the other. But whether the fjrees 

 be so near that anatomists cannot distingui>h muscles, cannot 

 point out muscles, resi-ting forces and balancing them — because, 

 remember, when you take a piece of glass in your fingers every 

 bit of pressure at every ten-thousandth of an inch pressed by the 

 glass against the finger is a balanced force — Dr whether they be far 

 asunder and obviously balanced tiy the muscles of the tu-o arms, 

 the thing perceived is the same in kind. Anatomi-ts do not 

 show us muscles balancing the individual forces exptrienced by 

 the small areas of the finder itself, when we touch a piece of 

 smooth glass, or the individual forces in the scores or hundreds 

 of little areas experienced when we touch a piece of rough sugar 

 or rough sandstone ; and perhaps it is not by muscles sujaller 

 than the muscles of the finger as a whole that the multitudinous- 

 ness is dealt with ; or perhaps, on the other hand, these nerves 

 and tissues are c jntinuous in their qualities with muscles. I go 

 beyond the range of my subject whenever I speak of muscles 

 and nerves ; but externally the sense of touch other than heat is 

 the same in all case- — it is the sense of forces and of places of 

 application of forces and of directions of forces, I hope now 

 I have justified the sixth sense ; and that you will excuse me for 

 having taxed your patience so long in not having done it in 

 fewer words. 



ELECTRICAL STANDARDS^ 

 n-HE Committee report that, in accordance with suggestions 

 made at the last meeting of the Briti?h Association, arrange- 

 ments have now been completed for testing resistance coils at 

 the Cavendish Laboratory and is-uing certificates of their value. 

 These arrangements have been made by Lord Kayleigh .md Mr. 

 Glazebrook, and the report contains an account by the latter of 

 the methods employed and the conditions under which the te.-ting 

 is undertaken, in order that those who use ;uch coils may have a 

 more exact estimate of the value of the test. 



When a coil is to be tested, a suitable st.andard is chosen, and 

 the two are placed in the water baths and left at least three or 

 four hours — more usually over night. The comparison is then 

 made in the ordinary manner by Prof. Carey F'oster's method 

 (Journal of the Society of Telegraph Engintirs, 1874), and the 

 coils again left for some time without being removed from the 

 water. After this .second interval another comparison is made. 

 The temperatures of the water baths are taken at each com- 

 parison, and as a rule differ very .slightly. 



* Abstract of Report of the Committee, consisting of Prof. G. Carey 

 Foster, Sir William Thomson. Prof. .A.vrton, \[r. J. Perrj-, Prof. W. G. 

 Adams. Lord Rayteigh. Prof. Jenkin. Dr. (1. T- L.dge, Dr. John Hopkin- 

 son. Dr. A. Muirhead (Se-retary). Mr. W. H. Preece, Mr. Herbert Taylor, 

 Prof. Everett, Prof. Schuster. Str W. Siemens. Dr. J. A. Fleming, Prof. 

 G. F. Fitzgerald, Mr. R. 1'. Glazebrook, and Prjf. Chrystal. appointed for 

 the purpose of constructing and issuing practic.^l Standards for use in Elec- 

 trical Measurements. 



We thus have two values of the resistance of the coil to be 

 tested at two slightly different temperatures. 



The mean of these will he the resistance of the coil in question 

 at the mean of the two temperatures. 



We are Ihus able to issue a certificate in the following form: — 

 "This is to certify that the coil No. .if has been comparel with 

 the British Association Standards, and that its value at a tem- 

 perature of A" C. is P B. A. Units or P' R. ohms ; I B.A. Unit 

 being 'gSd; R. ohms." We further propose to stamp all coils in 



the future with this monogram jK aiid a reference number. 



It will be noticed that nothing is said about the temperature 

 coefficient of the coil or the temperature at which the coil is 

 accurately I B.A. Unit. To determine this exactly is a some- 

 what long and troublesome operation, but at the same time it is 

 one which every electrician, if he knjws the value of the coil at 

 one given temperature, can perform for himself with ordinary 

 testing apparatus. It does not require the use of the standards. 

 For many purpoes the approximate value of the temperature co- 

 efficient obtained frojii a knowledge of the material of the coil 

 will suffice ; we may feel certain that any one requiring greater 

 accuracy v\ould be quite able, and would prefer, to make the 

 measurement himself. We can state with the very highest exact- 

 ness that the resistance of the coil X at a temperature A° C. 

 is R. To obtain the temperature coefficient accurately requires 

 an amount of labour which may he quite unnecessary for the 

 purpose for which the coil is to be used. 



In accordance with the resolution of the Committee, a fee of 

 l/. i.r. has been charged for testing single unit", and of i/. \is. 6d. 

 for others. 



The only coils the testing of which is regularly undertaken are 

 single units and multiples of single units by some powers of to. 



But though this is so, two standard ohms have been ordered, 

 using for the value of the B.A. unit "9867 ohms, and when they 

 arrive and have been tested, it will be easy to determine the 

 value of coils which do not differ much from a real ohm. At 

 pre-snt, wiihouc th^se stand.ud. — the coils actii.illy used in the 

 recent experiments at the Cavendish Laboratory have a resi-tance 

 of a'lout 'I, 24, and 168 ohms — the operation is troublesome. 

 The simplest accurate method seems to be to combine in multiple 

 arc the real ohm, and one of the too B.A. unit standards, and to 

 compare the combination with a single unit. 



ON THE MEASUREMENT OF ELECTRIC 

 CURRENTS 1 



OERHAPS the simplest way of measuring a current of mode- 

 rate intensity when once the electro-chemical equivalent of 

 silver is known, is to determine the quantity of metal thrown 

 down by the current in a given time in a silver voltameter. 

 According to Kohlr.ausch the electrochemical equivalent of 

 silver is in C.G.S. measure I '136 X lo"", and according to 

 Mascart, I'I24X lo"-. Experiments conducted in the Caven- 

 dish Laboratory during the past year by a method of current 

 weighing described in the British Association Report for 1882 

 have led to a lower number, viz. I'ligX lo-'-. At this rate 

 the silver deposited per ampere per hour is 4'028 grams, and 

 the method of measurement founded upon tliis number may be 

 used with good effect when the strength of the current ranges 

 from 1/20 ampere to pernaps 4 amperes. It requires, how- 

 ever, a pretty good balance, and some experience in chemical 

 manipulation. 



Another method, which gives good results and requires only 

 apparatus familiar to the electrician, depends upon the use of a 

 standard galvanic cell. The current from this cell is passed 

 through a high resistance, such as 10,000 ohms, and a known 

 fraction of the electromotive force is taken by touching this 

 circuit at definite points. The current to be mea-ured is caused 

 to flow along a strip of sheet German silver, from which two 

 tongues project. The difference of potential at these tongues is 

 the product of the resistance included between them and of the 

 current to be measured, and it is balanced by a fraction of the 

 known electromotive force of the standard cell (see figure). With 

 a sensitive galvanometer the balance may be adjusted to about 

 1/4000. The German silver strip must be large enough to avoid 

 heating. The resistance between the tongues may be 1/200 ohm, 

 and may be determined by a method similar to that of Matthies- 

 sen and Hockin (Maxwell's " Electricity," § 352). The propor- 



' Abstract of a paper read at the Cambridge Philosophical Society. 



