May I, 1891.J 



SCIENCE. 



249 



INDUSTRIAL NOTES. 

 Electrical Instruments for Schools. 



The immense spread of electrical invention and application has 

 required the services of many workers. It no longer suffices that 

 these workers be taken from other callings, or thrust out un- 

 trained from our grammar-schools and academies. They must 



be skilled not only in the theory, but in the actual use of instru- 

 ments and machines. But all cannot go to colleges and engineer- 

 ing schools to acquire this knowledge, and, even if they could, the 

 colleges have not time to go back to elementary principles and 

 teach the elementary use of instruments : hence a great part of 

 this work must be left to the high-schools and other preparatory 

 institutions throughout the country. Recognizing this fact, a 

 priori, as well as in consequence of many and repeated demands 

 from the schools themselves, Messrs. Queen & Co. have just de- 

 signed and placed upon the market a complete series of electrical 

 testing instruments for school use. Queen & Co.'s list of this 



apparatus embraces all the instruments needed for a full year's 

 course in laboratory electricity, and includes galvanometers of all 

 kinds, resistance-boxes, Wheatstone bridges, voltmeters, etc. We 

 note, from inspection of their catalogue, seven styles of winding 

 of simple horizontal galvanometers ; thus, the first one will meas- 

 ure currents from .01 to .5 of an ampere, and detect currents as 

 small as .0025 of an ampere. This galvanometer is illustrated in 

 Fig. 1. Fig. 3 shows a galvanometer which has, in addition to 

 the usual winding of wire, a heavy copper strip allowing the 

 measurement of currents up to 40 and 50 amperes. In addition 

 to these simple galvanometers, are several styles of fibre-suspended 

 galvanometers having an astatic system of needles and pointer 

 moving over a finely graduated circle, so that deflections can be 

 easily read. For still better work, the galvanometer shown in 

 Fig. 3 has been specially designed. This galvanometer is built 

 somewhat upon the plan of the well-known tripod galvanometer 

 of Sir William Thomson, and is extremely sensitive. The mirror, 

 which is very light, carries the magnetic system (several small bits 

 of steel) on its back, and the whole is suspended by a very fine 

 cocoon fibre about seven inches long. The coils are two in num- 



ber, and may be connected in series or in parallel, as desired. 

 They are easily movable. The magnetic system is enclosed by a 

 thin, plane glass in front, and by another similar one behind. The 

 latter is fixed in the end of a small tube which slips easily in the cen- 

 tral axis of the rear coil, so that the air-space may thereby be easily 

 increased or decreased at will. The galvanometer may in this way 

 be made dead-beat or used undamped, as desired. By pushing 

 the sliding tube until the air-space becomes small, readings may 

 be taken with great rapidity, as the mirror will come to rest very 

 quickly. The galvanometer may be made even more sensitive by 

 the use of a control-magnet arranged to slide upon the tube con- 

 taining the suspending fibre. This type of galvanometer is sup- 

 plied wound to resistances of 100, 800, or 3,000 ohms, according 

 to order. 



Fig. 4 is an illustration of a galvanometer which will also be 

 found useful. The coils, as in the last type mentioned, are two in 



number, and may be coupled up in series or multiple, as desired. 

 They are easily removable, and enclose a heavy block of copper 

 fixed in a central fork. This copper block has a small cylinder 

 bored partly through, in which hangs the bell-magnet making up 

 the moving system. The magnet, with mirror attached, is sus- 

 pended by a long and fine cocoon fibre, and, in consequence of 

 being enclosed in the copper block, comes to rest very quickly 

 after being deflected. In measuring and comparing condenser 

 capacities, electro-motive forces, battery resistances, etc., by con- 

 denser methods, this galvanometer is very good, for, by simply 

 lifting the copper block off the fork which supports it, the instru- 

 ment is made ballistic. The coils are held in place by a special 



FIG. 6. 



device, so that they may be readily changed for coils of other re- 

 sistances, thus adapting the instrument to almost all Tarieties of 

 galvanometer work. This galvanometer, like the preceding, is 

 furnished with any of several windings, or with several sets of 

 coils for the same instrument, thus making it applicable to meas- 

 urements of various kinds. 



