112 



SCIENCE. 



[Vol. Xn. No. 292 



current of 330 volts was turned on, and the dog was killed. 

 Further experiments were prevented by an agent of the Society for 

 the Prevention of Cruelty to Animals. This was unfortunate, as 

 the result was distinctly unfair to the alternating side of the ques- 

 tion, since a dog that had been subjected to a 1,000-volt contin- 

 uous current was hardly in condition to stand very much more, no 

 matter under what form the shock came. On Aug. 3, however, 

 the experiments were repeated on a number of dogs, before Dr. 

 Cyrus Edson and a number of physicians and electricians. The 

 main results are embodied in the letter to Science published Aug. 

 10. In this letter three cases are mentioned, in each of which a 

 strong and healthy dog was killed by alternating currents whose 

 voltage varied from 340.5 (the highest) to 234 (the lowest). In 

 some further experiments given by Mr. Brown in the Electrical 

 World, a number of dogs were killed by alternating and by con- 

 tinuous currents. The maximum alternating-current voltage that 

 was taken without death resulting was 500 ; continuous-current, 

 1,420 volts. Minimum alternating current that caused death was 

 at 188 volts; continuous current, 800 volts. The physicians 

 present expressed the opinion that a current which killed a dog 

 would be fatal to a man under similar conditions. 



If these experiments were conclusive, they would mean that al- 

 ternating currents would destroy life at less than half the voltage 

 that would make continuous currents dangerous. This is partly 

 due to the fact, that when we measure alternating electro-motive 

 forces by aCardew voltmeter, such as was used in Mr. Brown's ex- 

 periments, we measure the mean, not the maximum, electro-motive 

 force ; which last is, very roughly, half again as much. Still, as 

 we always consider the mean electro-motive force, and as the al- 

 ternating system uses a mean electro-motive force of 1,000 or 2,000 

 volts, whichever it may happen to be, we must drop the distinction 

 between mean and maximum, or we must carry it into our prac- 

 tical work. Both these experiments are contradicted by the state- 

 ments of various people that they have taken alternating electro- 

 motive forces as high as 1,000 volts without inconvenience. Still, 

 until these statements are more definite as to the conditions under 

 which the shocks were taken, we may consider that Mr. Brown 

 has the upper hand. 



But, whatever may be thought of Mr. Brown's e.xperiments on 

 dogs, they add to the evidence pointing to the fact that 'an alter- 

 nating current of 1,000 volts electro-motive force would be fatal. 

 Such a pressure is far above the limit set by M. d'Arsonval, and 

 few fair-minded persons will doubt its danger. This being the 

 case, the question arises. Should a system using such a pressure be 

 allowed for house-to-house distribution in crowded cities.? Under 

 certain circumstances, this could be safely done; under others it 

 could not. If the wires can be taken over head, and if the con- 

 verters may be placed on poles in front of the houses, the low- 

 pressure secondary circuit alone entering the house, then the 

 system, if properly installed, is reasonably safe, and should not be 

 objected to on that score. If it is necessary, on the other hand, to 

 put the wires under ground, and to bring them into houses to sup- 

 ply converters in the cellar, say, then the system is not safe, nor 

 will it be economical ; for the trouble and expense of keeping a 

 network of high-potential mains in order, leaving out the danger, 

 will take from the economy and popularity of the system. The 

 rational and safe way of using the system, in a city where overhead 

 wires are not allowed, is to have a number of sub-stations in the 

 district to be lighted, to which the high-potential mains are taken, 

 and from which current is distributed to the houses at a low 

 potential. On this latter plan, there is no reason that the system 

 should not be used in New York or anywhere else. The high- 

 potential conductors are less dangerous than the arc-light circuits , 

 for they are tapped at fewer places, and the current is not taken 

 into any house at a pressure high enough to cause death. It is, in 

 fact, the only safe method of alternating-current distribution under 

 ground. We hope that it will be tried, and that it will succeed. 



A New Electro-dynamometer. — M. Pellat has devised an 

 electro-dynamometer which seems sensitive, and whose constant 

 may be determined with accuracy from measurements. It consists 

 of two cylindrical coils of wire, one within the other. The axis of 

 the longer and larger one is horizontal, that of the smaller is ver- 



tical, and the two axes intersect at their middle points. If, now, a 

 current be sent through the coils, — the outer one being fixed, the 

 inner movable, — the axis of the latter will tend to place itself 

 parallel to that of the former. The smaller coil is at one end of 

 a scale-beam, and its tendency to move is balanced by weights 

 added to the pan at the other end. The current is calculated from 

 the weight in the pan and the dimensions of the two coils, the latter 

 being in the form of a constant. The current is conducted to the 

 inner coil by two silver wires joining the support with terminals on 

 the scale-beam, to which the ends of the coil are taken. The two 

 most difficult measurements that have to be made are the diameter 

 of the cylindrical coils and the distance apart of the turns of wire. 

 The former, M. Pellat states, can be made with an accuracy of i 

 part in 5,000 ; the latter, within i in 3,000 ; and, as we measure the 

 square of the current, the last error would only appear as i in 6,000 

 in the result. I n discussing all the sources of error, M. Pellat 

 reaches the conclusion that the results of measurements are correct 

 to at least i part in 2,000. The currents that can be measured are 

 not greater than .6 or .8 of an ampere, the difficulty lying in the 

 fact that in getting the current to the inner coil very fine wires 

 must be used in connecting the stationary with the movable parts, 

 otherwise the sensitiveness will be decreased. To allow for the 

 effect of the earth's magnetism, the current is sent first in one 

 direction, then in the other. The difference in the weighings is 

 due to the magnetism of the earth. In calculating the current 

 from a weighing, we have very simply i = A ^p, where ^ is a con- 

 stant calculated from the dimensions of the instrument, and p is 

 the weight in the scale-pan. The sensitiveness of the instrument 

 allows measurements to be taken within I in- 10,000. M. Pellat 

 proposes to use this instrument for calibrating other current- 

 measuring apparatus, for measuring electro-motive force (using^ it 

 in connection with a resistance), and for determining the horizontal 

 component of the earth's magnetism (employing it with a tangent 

 galvanometer). 



A Light-Weight Primary Battery. — In France M. Renard 

 has experimented for some time past on a navigable balloon. In 

 order to obtain the power necessary to direct it, he has attempted 

 to find an electric battery that is very light for its output, the ques- 

 tion of economy not entering. None of the primary or secondary 

 batteries in use would answer his purpose, and a new one had to 

 be invented. After a number of experiments, he found that the 

 best results were obtained by a cell in which the metals were zinc 

 and platinized silver, while the liquid was a mixture of hydrochloric 

 and chromic acids. The liquid is not very stable, but it can be 

 kept for several days if it is not exposed to light. There is no 

 local action in the cell if the chromic acid in the solution does not 

 fall below one-seventieth of the equivalent for the hydrochloric acid 

 present. The cells are made tube-shaped, the diameter being 

 about one-tenth of the height. The potential is 1.2 volts; and 

 from cells weighing thirty-three pounds, 200 to 250 watts per second 

 have been taken for two hours and a half. For a storage-battery 

 of the same weight, the energy could not be taken out at a rate of 

 more than 75 or 100 watts, and this at a low , efficiency. 



MENTAL SCIENCE. 



The Effect of Practice upon Reading. 

 lN.\SMtJCH as all education is in essence mind-building reduced 

 to an art, the strictly psychological study of mental phenomena 

 must in the end yield results of high practical import. It is this 

 conviction that has brought the psychologist and the educator into 

 such close sympathy, and has brought the latter to eagerly await 

 the results of the former's somewhat specialized and technical 

 studies. To no topic is this more applicable than to the study of 

 the times taken up by various simple psychic processes, and of the 

 causes influencing such times. We here touch upon the very 

 powers that the teacher aims to develop, and, if we can acquire a 

 method of testing these powers, we are sure to learn more of their 

 real nature. A very promising contribution in this direction has 

 been recently published by Dr. G. O. Berger {Philosophzsche Stic- 

 dien. v. I), an account of which will probably be of interest to 

 American students. 



