June 7, 1888] 



NA TURE 



141 



opposition between them is in accordance with a principle 

 applicable in common to the excitable structures of plants and 

 animals, viz. that the property which renders a structure capable 

 of undergoing excitatory change is expressed by relative posilivity, 

 the condition of discharge by relative negativity. 



The fourth section of the paper is devoted to an investigation 

 made in 1887, of the events of the first second after excitation, 

 made with the aid of a pendulum-rheotome specially adapted for 

 the purpose. The fifth contains the description of the records 

 obtained by photographing the electrical phenomena of the 

 excitatory reaction, as observed with the aid of the capillary 

 electrometer, on rapidly-moving plates. Both of these series of 

 observations serve to confirm and complete the results obtained 

 by other methods. The photographs were exhibited. 



Physical Society, May 12. — Prof. Reinold, F.R.S., Pre- 

 sident, in the chair. — The following papers were read : — 

 Note on the condition of self-excitation in a dynamo machine, 

 by Prof. S. P. Thompson. It is a well-known fact that a series 

 dynamo running at a given speed will not excite itself unless 

 the resistance is less than a certain value, depending on the speed 

 and construction of the machine, and if the resistance is slightly 

 less than this critical value the excitation will not be such as to 

 saturate the magnets. According to the primitive statement of 

 the action of self-exciting dynamos on the " compound interest 

 law, "a dynamo should excite itself to saturation at any finite speed 

 providing the resistance is not infinite. An explanation of the 

 observed facts is given in the paper, without any assumption as 

 to the curve of magnetization. If E = E.M.F. of the machine, 

 n = speed, C = number of wires on outside of armature, 

 N = number of magnetic line-, i = current, S = number of 

 turns on magnet, 2R and 2p the sums of the electric and mag- 

 netic resistances respectively, then E = «CN, i = «CN/2R, and 

 N = 47rS//2p. From these it is easily seen that 4ttmCS = 

 2p . 2R, (A). ; i.e. for a dynamo running at constant speed the 

 product of the magnetic and electric resistances is constant, 

 and the dynamo will not excite itself if 2R is greater than 

 4ir«CS/2p. Similarly for a given value of 2R, excitation is im- 

 possible if n is less than 2p . 2R/4TCS. For a value of 2R less 

 than the critical value the excitation increases until the mag- 

 netic resistance is increased so that equation (A) is satisfied. 

 The corresponding formula for shunt machines is ^irnCZ — 



fp{(r a 



+ *> + Sp 



; where Z = number of shunt turns ; r a 



r s , and R, the resistances of armature, shunt, and external circuits 

 respectively. In the discussion which followed, Mr. Kapp de- 

 scibed a method used in testing dynamos, for determining 

 the minimum speed at which dynamos will excite themselves, 

 and from thence determining the magnetic resistance of the air 

 gap. In all cases experiment showed this to be less than the 

 calculated resistance, generally in the proportion of 1500 to 

 i860, the difference being greater in low-tension machines. 

 Prof. Ayrton pointed out that permanent magnetism was not 

 taken into account, and that the apparent resistance due to self- 

 induction, and between the brushes and commutator were con- 

 siderable for small currents. Lord Rayleigh and Sir W. Thomson 

 had shown critical speeds for given resistances to exist in 

 Faraday's disk dynamo. He (Lord Rayleigh) did not approve 

 of the term "magnetic resistance," and thought "reluctance," 

 as recently suggested by Mr. Heaviside, would be preferable. — 

 Note on the conditions of self-regulation in a constant potential 

 dynamo machine, by the same author. In "Dynamo-Electric 



Z r 



Machinery" a formula — = - — is given as expressing the 



S I'a + r m 



ratio of the number of turns in the shunt and series windings of 

 a compound dynamo. This is on the assumption that there is 

 no saturation within the working limits. As this assumption is 

 not legitimate, a correcting factor is necessary. The factor is 

 shown to be the ratio of the average ptrmeability over the 

 whole working range to the permeability corresponding with 

 no external current. The formula is transformed so as to be 

 expressed in terms of the " satural " data of the machine, which, 

 as shown in a previous paper, can be calculated from its details. 

 — On magnetic lag, and the work lost due to magnetic lag in 

 alternating current transformers, by Mr. Thomas H. Blakesley. 

 The method adopted to detect the lag is to place dynamometers 

 in both circuits, and one with a coil in each. Then, on the sup- 

 position that the E.M.F. of the secondary circuit is entirely due 

 to the changing magnetism of the core, the author proves that 



the tangent of the magnetic lag angle must be equal to 

 — Ca 3 - Ba 2 



• where m and u are the number of 



m I 



(AB« A - CV) 



turns in the primary and secondary coils respectively ; A, B, C, 

 the constants of the dynamometers ; and a u a 2 , o 3 , their angular 



reading. A is such that Aa t = _L , where I x is the maximum 



2 

 value of the primary current. A table of actual results is given, 

 where the magnetic lag is about Sz°- The whole power given 



out by the machine takes the form t^Ac^ + r 2 — C</ 2 , where r, 



n 

 and r 2 are the resistances of the primary and secondary 

 circuits, while the power lost in hysteresis is expressed by 



r 2 ( — Ca 3 - Bo 2 J. The lag is attributed to an induced magnetic 



stress called into being by the increasing or decreasing magnetism 

 itself, and always opposing it as motion in a medium induces 

 an opposing force of friction. By supposing such an induced 

 magnetic stress in quadrature (as Mr. Blakesley expresses it) 

 with the magnetism, and of such a value as when compounded 

 with the stresses due to the currents shall bring the resultant 

 into quadrature with the secondary current, the effective mag- 

 netic stress is obtained. This involves a new idea called 

 magnetic self-induction with its coefficient. The whole problem 

 is treated by the geometrical method, which the author has 

 applied to several other problems in alternating currents. Mr. 

 Kapp, Profs. Thompson, Perry, and Ayrton, and Lord Rayleigh 

 took part in discussing the paper. — On a simple apparatus for 

 the measurement of the coefficient of expansion by heat, by 

 Prof. W. E. Ayrton, F.R.S., and Prof. J. Perry, F.R.S. The 

 apparatus consists of a metal tube, within which the wire or 

 rod whose coefficient is to be determined is placed. One end 

 of the wire is rigidly attached to one end of the tube, and the 

 other end connected to an Ayrton and Perry magnifying spring, 

 a pointer attached to which indicates the change of length due 

 to alteration of temperature. Steam or water may be passed 

 through the tube, the temperature of the wire being shown on a 

 thermometer. The arrangement is very sensitive, and with a 

 pointer about 20 cm. long, the motion is magnified about 

 1000 times. — A magnifying spring attached to an aneroid was 

 also shown, and its great sensibility demonstrated. A com- 

 bination of a spring of large diameter and pitch with one of 

 small diameter and pitch was exhibited. By such a combination 

 small rotations can be immensely magnified. The great features 

 of the patent spring as a magnifier are the entire absence of 

 friction and back lash, and the large range of proportionality. 



Chemical Society, May 17.— Mr. W. Crookes, F.R.S., in 

 the chair. — The following papers were read : — Researches on the 

 constitution of azo- and diazo-derivatives ; (iv.) diazo-amido-com- 

 pounds, by Prof. Meldola, F.R.S. , and Mr. F. W. Streatfield.— 

 The colour of some carbon compounds, by Prof. Carnelly, and 

 Mr. J. Alexander. An investigation of a number of metallic 

 derivatives of ortho- and para-nitrophenol has given the following 

 results: (1) in all cases without exception the colour passes 

 towards the red end of the spectrum as the temperature rises ; 

 (2) the colour of the ortho-derivative is nearer the red end than 

 that of the corresponding para-compound ; (3) a comparison of 

 the nitrophenates of the metals belonging to the same sub-group 

 shows that the colour passes towards the red end as the atomic 

 weight of the metal increases ; (4) when the same salt occurs 

 in both the anhydrous and the hydrated state, the colour passes 

 towards the red end as the quantity of water of crystallization 

 diminishes ; (5) as regards the salts investigated, the para- 

 compound always takes up a larger quantity of water of 

 crystallization than the corresponding ortho-compound. In the 

 course of the discussion which followed the reading of the paper, 

 Prof. Armstrong, F.R.S., remarked that the facts advanced were 

 far too few to justify the very general conclusions arrived at by 

 the authors ; all who had worked with the nitrophenols were well 

 aware that the colour changed on heating in the manner 

 described ; and there was no novelty in the statement that the 

 para-nitrophenols crystallized with the larger proportion of water. 

 Referring to the authors' fourth deduction, he quoted calcium 

 parachlorodiorthonitrophenate as an exception, since this com- 

 pound can be obtained either in yellow anhydrous crystals, or in 

 deep-orange hydrated crystals. — The identity of natural and 



