NATURE 



[March 27, 1913 



10 per second, the lag is inappreciable, but for con- 

 siderably faster movements it becomes important. 



In duplex working when the sending current has 

 to be balanced so as not to affect the receiver, quick, 

 "jarry" movements are very difficult to eliminate, 

 but the lag in the thermo instrument reduces these 

 movements very considerably and is a valuable pro- 

 perty. 



When the thermopile is in its central position and 

 no current is flowing both junctions are at a dull red 

 heat, and when fully deflected one junction becomes 

 briq-ht red and the opposite one is black or very faintly 



I microamperes. 



red. In intermediate positions the current generated 

 by the thermopile is nearly proportional to the deflec- 

 tion. 



The curve (Fig. 4) was taken from a thermopile 

 with seven junctions on each side. When the thermo- 



pile was deflected 0075'' the current it sent through 

 a resistance of 42 ohms (equal to its own resistance) 

 was 081 milliampere. With the natural period of the 

 coil equal to 8-7 per second and a 4So-ohm 4So-turn 

 coil, a current of 003 milliampere through the coil 

 gave a current of 08 1 milliampere from the pile 

 through an external resistance of 42 ohms. For 

 slowly changing currents this corresponds to a mag- 

 nification of power of about twenty-seven times, and, 

 of course, this can be greatly increased by reducing 

 the period of the coil. For quickly changing move- 

 ments the power magnification is not so great, owing 

 to the back E.M.F. of the coil. 

 NO. 2265, VOL. 91] 



Trials of this instrument on an Atlantic cable have 

 shown an increase in speed of about 40 per cent. 



Mechanical Relay. 

 The instrument just described is a magnifying relay 

 — that is to say, it multiplies the impulses received 

 in exact proportion to their strength. This form of 

 relay is quite distinct from an ordinary make-and- 

 break relay, which delivers a constant current for any 

 impulse over a certain strength. For very many pur- 

 poses it is essential that received impulses should be 

 magnified without altering their shape, and this can 

 only be done by an instrument with a con- 

 stant magnifying power. 



That this is the case in the thermo relay 

 is shown by the diagram (Fig. 41, where 

 the current supplied to the coil and th cur- 

 rent delivered by the thermo-junctions are 

 plotted. Within the range of the instrument 

 the points lie on a straight line and repre- 

 sent, in this case, a constant magnification 

 in current of about twenty-seven times. 



This property I will now illustrate in an 

 entirely mechanical relay in which movemi nts 

 d by very small forces are largely 

 increased in strength without affecting their 

 motion. The relay consists in principle of 

 a rotating- spindle around which are wound 

 one or more turns of a flexible cord. The 

 spindle is revolving in such a direction as 

 to pull away from the magnified forces and 

 towards the small forces that control the 

 movement. Suppose a heavy weight has to 

 be raised by a force of one-tenth of the 

 amount, it will obviously be necessary to 

 supply 90 per cent, additional energy, and 

 this is supplied by the motor driving the spindle. 

 The magnification of force and energy depends on 

 the number of turns which the curd makes 

 round the spindle and follows a compound interest 

 law. 



In the model shown it will be seen that a 

 large magnification of power can be easily 

 obtained by very simple means. Thus I can 

 move this 14 lb. weight rapidly up and down 

 by pulling upon this silk fibre. 



Fig. 5 shows an application of the principle 

 to cable work, in which the small forces 

 operating the coil A are intensified sufficiently 

 to work the coarse relay arm R. The spindle 

 rotates away from the relay arm R and to- 

 wards the coil, and produces a much greater 

 tension in the fibres t than in s. When the 

 coil swings on its axis the tension is increased 

 in one of the fibres and diminished in the 

 other, and a similar change in a magnified 

 degree takes place in the fibres t. 



By using means of this sort it is possible 

 to work an ordinary siphon direct writer which 

 normally requires some 3 milliamperes by a 1 urrenl 

 of 10 microamperes. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 A Summer School in Geography will b ■ held at the 

 University College of Wales, Aberystwyth, on July 

 28-August 16. Among the subjects included in the 

 scheme of work are : — Human geography. Prof. H. J. 

 Fleure; climatology and trade routes, W. E. White- 

 house; land forms and natural regions, E. S. Price; 

 field classes and excursions. 



