5 o8 



NATURE 



{Sept. 2 1, l882 



one wire to the other, and the destruction of the wires. 

 Again the ends of the wires are almost sure when removed 

 from the instrument to be left dangling either in contact, 

 or so as to be easily brought into contact inadvertently 

 by a passer by, with the certain result if the dynamo is 

 running, of the immediate fusing of the wires. To pre- 

 vent the possibility of such an accident Sir William 

 Thomson has used as terminals for the coil two 

 strong strips of copper about \\ cms broad which stand 

 up vertically facing one another about a centimetre apart, 

 within a vertical cavity in the wooden block behind the coil! 



To prevent any current from flowing through the coil ex- 

 cept when a reading is being taken, the small spring contact 

 key, shown behind the coil in Fig. 1, is inserted between one 

 of these terminals and the coil. The leads for connect- 

 ing the instrument with the circuit have their ends brought 

 together so as to terminate in two parallel strips of stout 

 copper kept apart by a piece of wood and held in posi- 

 tion by a good serving of strong waxed cord. The two 

 copper strips with the piece of wood between them have 

 their ends turned down at right angles to their length, 

 and when connection is to be made are pushed down into 



the cavity between the two bars to which the ends of 

 the coil are attached. These bars are placed sufficiently 

 near together to be forced a little apart by the contact 

 piece, and thus give a secure spring contact. The leads are 

 made of thin stranded copper wire, well protected by a thick 

 woven covering of cotton, and are very flexible. They ter- 

 minate in two spring clips (shown in Fig. 1 ) made each of a 

 strip of stout copper held firmly against the flat side of a 

 piece of wood, of semicircular section, by an india rubber 



band passed round a groove in a semicircular piece of 

 brass soldered to the copper strip, and round the back of 

 the piece of wood. A groove carried along the piece of 

 wood above the elastic band prevents the copper strips 

 from turning round relatively to the wood, and thus a 

 good and safe contact is made between the copper and 

 anything on which it may be clipped. These clips are 

 quite as efficient as binding screws and a great deal 

 more convenient. They can in an instant be attached to 

 or removed from a wire or lead of any size and in any 



position. For convenience in the use of the instrument 

 the covering of one lead is coloured red, of the other blue. 



II. The Current Galvanometer. 



This instrument is shown in Fig. 3. It differs from 

 the galvanometer above described only in the coil and 

 arrangement of terminals. The coil is made of stout 

 copper strip about 17 cm. broad and 15 mm. thick, 

 wound in six turns, insulated by asbestos paper placed 

 between. The outside diameter of the coil is about 

 iocm.,the inner diameter about 6cm. It is covered like the 

 other with silk ribbon, and attached in a similar manner 

 to the platform P. A magnetometer exactly the same as 

 that described above is used with the instrument, and all 

 that has been said above with regard to the graduation of 

 the potential galvanometer is applicable also in the 

 present case, except that now amperes, not volts, are the 

 subject of measurement. 



This instrument is of course only suitable for the 

 measurement of continuous currents, but owing to the 

 small resistance of the coil, it can be left without risk of 

 damage in a circuit with a current of upwards of 100 am- 

 peres flowing continually through it, while it is of suffi- 

 cient sensibility to measure with accuracy, when the 

 needle is acted on by the directive force of the earth 

 alone, a current of from i-ioth to i-iooth of an ampere. 



In special instruments for measuring very strong cur- 

 rents the coil is made of a single turn of massive copper 

 strip, fitted with proper terminals to obviate undue 

 heating at the contacts. With this mode of construction, 

 an instrument can be made which shall measure with 

 accuracy currents of from i-ioth of an ampere to 1000 

 amperes. 



A pair of well-insulated leads several yards long, made 

 of copper- wire cable containing 133 strands of wire of 

 ■32mm. diameter (No. 30 B.W.G.), and therefore very 

 flexible and of inappreciable resistance, are sent out with 

 each instrument to be used with it. These are shown 

 coiled on the table beside the instrument in Fig. 3. 



The terminals of the instrument and the mode of 

 including it, by means of its leads, in any circuit 

 in which it is to be used, are worthy of a little attention. 

 In order that the galvanometer may be used to measure 

 the currents in different circuits, it must be introduced 



