294 



SCIENCE. 



[Vol. XIII. No. 324 



laboratory in which the ammeters and voltmeters are calibrated, we 

 find some very important points about the D'Arsonval galvanometer. 

 Numerous attempts have been made to use this form of galvanom- 

 eter as direct-reading ammeters and voltmeters, but the employ- 

 ment of permanent magnets makes their instrument one of only a 

 very limited degree of permanent accuracy. The same principle 

 is employed by Sir William Thomson in his siphon recorder ; and 

 Messrs. Deprez and D'Arsonval, and many others, have tried to 

 make commercial and portable instruments, as well as delicate 

 laboratory instruments. All these instruments have the same 

 defect, that their sensibility diminishes as the magnets grow 

 weaker. Besides this serious trouble, further investigations have 

 shown, that, in spite of the very small angles through which the 

 coil moves, the deflections will not be proportional to the cur- 

 rent. Ayrton describes this as follows : " If you start from the 

 centre, so that the spot of light is at one extreme end of the scale 

 for no current, you find, on carefully calibrating the instrument, 

 that you get a broken line consisting of 'wo straight lines meeting 

 at an angle, or probably, strictly, meeting according to some curve 

 at about the spot corresponding with the plane of the coil, being 

 parallel to the lines of force. Hence there is a difficulty in dividing 

 the scale uniformly ; and this difficulty is met with even when the 

 plan of using curved pole-pieces is adopted, as proposed by us 

 some six years ago." 



The most interesting instrument, however, brought out and per- 

 fected by Professors Ayrton and Perry, is the new direct-reading 

 hot-wire volt- and ammeter. The underlying principle is that which 

 is used by Cardew in his voltmeters, — that the passage of a cur- 

 rent will heat a wire, and thereby lengthen it. Ayrton and Perry 

 had an excellent means in their patent springs of multiplying the 

 minute changes of the dimensions ; and the employment of this 

 very spring reduces the Cardew of three feet length and four yards 

 of fine wire to one in which eight inches of wire are sufficient to 

 indicate differences of potential of less than f^^j of a volt. The 

 above figure shows a cross-section of the instrument as at present 

 constructed. The combined pull of the spring ^/ and of the plati- 

 num silver wire WW, attached to the blocks B, B, is counterbal- 

 anced by the pull of the spring S. Hence, as the wire stretches, 

 the magnifying spring M is stretched, and the point P, to which a 

 number of fine hairs are attached to introduce damping without 

 solid friction, rotates. The flat spring is not only introduced to 

 enable the depth of the instrument to be diminished by twice the 

 sag of the wire, but to enable a particular arrangement of fuse to 

 be employed. 



The fuse F is of such a diameter that it would require a far 

 larger current to melt it than would damage the instrument. In 

 addition, a platinum-tipped screw, D, is arranged so that when the 

 wire ^ fK stretches by any pre-arranged percentage beyond the 

 amount it stretches for the maximum safe potential difference, 

 the platinum tip C, of the flat spring S. comes into contact with 

 the screw D, and the working wire is short-circuited. The circuit 

 is then temporarily completed through the lead Z, to the left of the 

 flat spring S, and the fuse F, when the current increases and the 

 fuse melts without damage to the instrument. With this device, 

 they find that the fuse may be thick, and therefore have but a small 

 resistance compared with that of the working wire, and yet the 

 sudden application of a potential difference five or six times as great 

 as the maximum potential difference the voltmeter is intended to 

 measure, melts the fuse without damaging the working wires. For 

 clearness, the fuse, the key, and the terminals V, V, are shown de- 

 tached from the instrument, but they are in reality in the base, as 

 seen in Fig. I. 



AN ELECTRIC DOOR-OPENER. 



An improved electric door-opener, manufactured by Wendt & 

 Co., of this city, is shown in the accompanying illustration. It re- 

 quires but small battery power. The wires are concealed from 

 view ; and a slight pressure on the ordinary electrical push-button 

 not only releases the latch, but causes the door to be thrown open. 

 The closing of the door resets the opener automatically. 



The mechanism of the door-opener is simple, i? is a throw-back, 

 which swings on a pivot to which is attached the inner end of a 



coiled flat spring. To a crank on the end of the pivot, outside the 

 coiled spring, is attached a spiral spring. The closing of the door 

 swings the throw-back around on its pivot, thereby compressing the 

 coiled spring, which is of sufficient strength to force open a heavy 

 door when released. The spring is prevented from acting until 

 the armature, which is pivoted at the lower right-hand corner of 

 the opener, is attracted by the electro-magnet. The movement of 

 the armature allows free movement to a tongue pivoted to the up- 



per left-hand corner of the electro-magnet, against which the latch- 

 bolt lever bears. The action of the opener is as follows : When 

 the circuit is closed by a touch on the button, the armature is at- 

 tracted by the electro-magnet ; the tongue moves, thereby releas- 

 ing the spiral spring, which pulls back the latch-bolt ; and at the 

 same time the coiled spring forces the door open by means of the 

 throw back. 



Thomas Whittaker has just published " The Washington 

 Centennial Souvenir," — a large, finely printed, and handsomely 

 illustrated brochure arranged by Frederick Saunders of the Astor 

 Library. 



