Profs. Ayrton and Perry lately described and exhibited before the Physical Society their new ammeters and voltmeters, also a non-sparking key. The well known ammeters and voltmeters of the authors used for electric light work are now constructed so as to dispense with a constant, and give the readings in amperes and volts without calculation. This is effected by constructing the instruments so that there is a falling off in the controlling magnetic field, and a considerable increase in the deflecting magnetic field. The deflections are thus made proportional to the current or E.M.F. measured. The ingenious device of a core or soft iron pole-piece, adjustable between the poles of the horseshoe magnet, is used for this purpose. By means of an ammeter and voltmeter used conjointly, the resistance of part of the circuit, say a lamp or heated wire, can be got by Ohm's law. Profs. Ayrton and Perry's non-sparking key is designed to prevent sparking with large currents. It acts by introducing a series of resistance coils determined experimentally one after the other in circuit, thereby cutting off the spark. * * * * * NEW INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS AND ELECTRO-MOTIVE FORCE. By Messrs. R. E. CROMPTON and GISBERT KAPP. [Footnote: Paper read before the Society of Telegraph Engineers, 14th February, 1884.] In consequence of the rapid development of that part of electrical science which may be termed "heavy electrical engineering," reliable measuring instruments specially suitable for the large currents employed in lighting and transmission of energy have become an absolute necessity. As usual, demand has stimulated supply, and many ingenious and useful instruments have been invented, the manufacture of which forms at the present day an important industry. Mr. Shoolbred, in a paper which he recently read before this Society, gave a full and interesting account of the labors of our predecessors in this field. To-day we add to the list then given a class of instruments invented by us, examples of which are now before you on the table. We have preferred to call them current and potential indicators in preference to meters, considering that the latter term, or rather termination, ought to be applied rather to integrating instruments, which the necessities of electric lighting, we believe, will soon bring into extensive use. The principal aim in the design of these indicators has been to obtain instruments which will not alter their calibration in consequence of external disturbing forces. If this object can be attained, then it will be possible to divide the scale of each instrument directly into amperes or volts, as the cause may be, and thus avoid the use of a coefficient of calibration by which the deflection has to be multiplied. This is an important consideration when it is remembered that in many cases these instruments have to be used by unskilled workmen, to whom a