August 21, 1883. 



SCIENCE, 



153 



in its axis, moves ladially over a disk upon which is 

 the graduation. The disk can be turned through a 

 small arc for the adjustment of the zero. The instru- 

 ments are very compact, simple, and strong, and 

 very convenient in use; but the spring seems some- 

 what more likely to change than the simple spiral 

 used by Kohlrauscb. 



Instead of the spring, Hopkins has proposed to use 

 mercury confined in a capsule, the bottom of which 

 is formed by an elastic diaphragm, upon which the 

 iron core exerts a pall wben the current passes. The 

 capsule is filled with mercury, as is also a portion of 

 a vertical glass tube inserted into it. The stress ex- 

 erted upon the bottom of the capsule causes the mer- 

 cury to fall in tlie tube, which may be provided with 

 a scale, indicating current strength in the customary 

 units. Various forms of apparatus recently de- 

 scribed, involve the same or similar principles, using 

 an iron rod floating in a cylinder partly filled with 

 mercury, and an index-tube in which tlie mercury 

 moves as displaced by the iron core, or having in the 

 index-tube a lighter liquid for the purpose of increas- 

 ing the range. 



An entirely novel device has been employed by 

 Lippmann. A horizontal tube, bent upward at the 

 two ends, and partially filled with mercury, is placed 

 between the poles of a strong magnet. By means of 

 conducting-wires, the current is conveyed through 

 the mercury in a vertical direction, at a point in the 

 space between the poles of the magnet. The mer- 

 cury, traversed by the current under the action of the 

 magnet, is subject to a force which tends to move it 

 laterally, thus changing the level in the two vertical 

 arms, by an amount which is proportional to the in- 

 tensity of the current. As the sensibility requires 

 that the quicksilver column where traversed by the 

 current should be very thin, this portion of the tube 

 is given the form of a flat chamber only a fraction of 

 a millimetre in thickness. If used for strong currents, 

 the heating of the mercury would take place rapidly, 

 and cause serious inconvenience. To avoid this 

 would necessitate making the apparatus in much 

 larger dimensions, with a loss of sensitiveness, or 

 shortened range. 



The rotation of the plane of polarization of a ray of 

 light under the influence of an electrical current has 

 been proposed as a means of measuring the current. 

 Experiments, by a number of physicists, have shown 

 that measurements may be made with considerable 

 accuracy in this way; but as they all depend upon the 

 determination of a plane of polarization, the device 

 is found to be less convenient in its application than 

 other methods. 



We may notice in passing Cardew's volt-metre, in 

 which the current is measured by the extension of a 

 wire heated by it, an idea, which, though not new, 

 has been applied to form a practical and useful in- 

 strument. 



In all the instruments in which the current to be 

 measured produces motion of a needle, or of a portion 

 of the circuit, the action of external magnetic forces, 

 whether of tiie earth, or of the machinery and cir- 

 cuits, as has already been noted, would be felt as 



soon as the sensitiveness of the instrument is pushed 

 to the point required for great accu!acy, and would 

 make special provisions and precautions necessary. 

 The spring instruments, as they utilize not the direc- 

 tive, but the attractive or repulsive action of the 

 circuit, are almost entirely free from such disturb- 

 ance, and are therefore better suited for those cases 

 where time cannot be given to preliminary experi- 

 ments for adjustment, and the determination of con- 

 stants, or where it is desired to follow the changes of 

 a rapidly varying current. It must be noted, more- 

 ever, as has been recently pointed out by Hospitalier, 

 that where the changes in the current occur too rap- 

 idly, and especially in Ihe case of intermittent or 

 alternating currents, the self-induction in the coils 

 of these instruments may give rise to considerable 

 errors in their indications; and also that in all those 

 cases where the effect to be measured depends upon 

 the square of the current strength, instruments act- 

 ing upon the principle of the dynamometer must be 

 used to obtain trustworthy resuhs. 



Arthur W. Wright. 



INCANDESCENT LAMPS ON RAILWAYS. 



For several months past, the Pennsylvania railroad 

 company have been lighting nine of their cars with 

 incandescent electric lamps. The electricity is pro- 

 duced by Brush storage batteries, which are charged 

 once a week. The storage battery is carried under- 

 neath the cars in boxes built to receive them, — one- 

 half being placed on each side. Each car requires 

 six trays of four cells each. The trays are made so 

 that the simple process of putting the trays in posi- 

 tion completes the electric circuit. The battery 

 when charged has an electro-motive force of forty- 

 five volts; and, when the electro-motive force has 

 fallen to thirty-nine volts, the battery is recharged. 

 The batteries are charged at the depot in Jersey City 

 by a sixteen-light Brush machine. In charging, the 

 ordinary Brush manipulator, without the register, is 

 employed. 



Swan lamps consuming 1.1 amperes have been 

 used almost exclusively, although Stanley-Thom- 

 son's lamps have been tried. The parlor-cars require 

 ten sixteen-candle-power lamps, while the passenger- 

 cars require but six. The lamps are all in parallel 

 circuit, and so arranged that one-half may be used 

 at a time. The wires are led through a clock mech- 

 anism, which registers the time they have been 

 used. By an ingenious mechanical device, the clock 

 is made to move half as fast when the switch throw- 

 ing off half the lamps is turned. 



Altogether, some seventeen batteries, of twenty- 

 four cells each, are in use; and, as yet, only one cell 

 has been disabled. As to loss of eflQciency, due to 

 deterioration, no tests have been made. Although 

 the lamps are probably much less than sixteen-can- 

 dle power, it is probable that their life is less than 

 that of those used in buildings, because of the jarring 

 to which they are subjected. 



