98 



SCIENCE. 



[Vol. XIII. No. 314, 



spring voltmeters and ammeters of Ayrton & Perry. These are, 

 however, subject to serious heating and frictional errors. 



Thus far we have mentioned only the most glaring defects com- 

 mon to commercial instruments. Another and almost universal 

 defect is due to inaccuracies in the reading-scale. A great ad- 

 vance was certainly made when direct-reading instruments were 

 substituted for those requiring a multiplying constant. It is a 

 notable fact, however, that most of the direct-reading instruments, 

 judged by the uniformity of their scale-divisions, follow the propor- 

 tional law. This is extremely doubtful, however, and calibration 

 generally reveals the fact that seldom are there more than two or 

 three of the scale-marks correct. Printed or engraved scales may 

 justly excite suspicion as to their accuracy. 



The advent of an improved type of commercial electrical measur- 

 ing instruments for direct-current circuits, in which the sources of 

 error enumerated above are practically eliminated, and which are 

 equally well adapted for both laboratory standards and commercial 

 service, is justly to be regarded as an important step in the develop- 



nary use, is now generally acknowledged. In these instruments 

 special care has been taken not only in the selection of steel and its 

 proper magnetization and artificial aging, but the magnetic resist- 

 ance of the acting field has been reduced to its lowest practical 

 limit by the insertion of a central core of soft iron within the mova- 

 ble coil. This core is supported upon the magnet frame by a strap 

 of diamagnetic material. The form of the pole-pieces is such that 

 the deflecting coil constantly moves in a uniform field, and hence 

 the deflections practically follow the proportional law ; and a direct- 

 reading scale, of nearly equal subdivisions, is obtained. The mova- 

 ble coil is wound upon a light frame of copper, which serves the 

 double purpose of a support, and also, since it moves in an intense 

 field of force, as a damper, thus making the instrument exceedingly 

 dead-beat. 



All of the more important parts of the instrument are made 

 to gauge, and the bearings of the deflecting coil are jewelled. Inter- 

 changeability of parts, and the elimination of friction, are thus ob- 

 tained. A difference of potential of about one and one-quarter 



inent of electrical engineering. The Weston Electrical Instrument 

 Company of Newark, N.J., have recently placed upon the market a 

 new form of commercial voltmeters and ammeters, designed by 

 Edward Weston, the well-known electrician. The especial aim in 

 designing these instruments has been not only to entirely eliminate 

 such variable factors as have been enumerated above, but to add 

 to the instrument certain valuable features mentioned below. To 

 this end, the electrical, magnetic, and mechanical features have 

 been so worked out as to insure permanence and reliability, 

 coupled with simplicity, extreme accuracy, a wide range of scale- 

 reading, and portability. The accompanying engravings (Figs. I, 

 2, and 3) give general and detailed views of the voltmeter. It will 

 be seen that the field of force is produced by a permanent magnet 

 of peculiar form, while the deflecting body, carrying the index, is a 

 light coil of insulated wire, whose motion, resulting from the dual 

 fields established by the magnet and current circulating in the coil, 

 is restrained by two coiled springs. The springs serve also to con- 

 vey the current into and out of the moving coil. 



That permanent magnets can be, and in fact are, daily made, 

 which, after undergoing a process of artificial aging, remain 

 thenceforth practically constant in strength when subject to ordi- 



volts, at the terminals of the movable coil, serves to deflect the in- 

 dex over the entire scale, the length of the scale being about six 

 inches. This degree of sensitiveness permits, therefore, the con- 

 struction of instruments having a wide range of maximum scale- 

 reading by the simple insertion of differentially wound resistance- 

 coils, in series with the movable coil. 



Figs. I and 2 exhibit the external and internal parts respectively 

 of the new voltmeter, having a scale of double values, with a ratio 

 of 20. The scale divisions for the upper values (Fig. I) are single 

 volts, while for the lower values they read one-twentieth of a volt. 

 The single divisions are of such a size that one can easily read to 

 one-tenth of a division ; namely, to one-tenth of a volt on the upper 

 values, and one two-hundredth of a volt on the lower. This form 

 and ratio of scale values is useful for battery-work, and especially 

 for storage-batteries. The lower scale values are used when ex- 

 amining single cells, or sets of two ; and the upper scale values, 

 when measuring the aggregate potential difference of a series of 

 cells. In the former case, connection is made with the small bind- 

 ing-post (under the rubber cap) on the left, and with the larger 

 binding-post on the right. In the latter case, connection is made 

 with the two large binding-posts. 



