144 



NATURE 



[December 6, 1900 



of 1000 mm. is nearly 600 mm. per ampere. The other ter- 

 minals which are seen make connection with the magnet, on 

 which there are four pairs of coils the ends of which are brought 

 out to separate terminals so that the coils may be connected up 



Fig. 4. — Photographic recording arrangement. 



in series or parallel to suit the voltage obtainable. The magnet 

 is wound so that the coils may be connected in series direct 

 across 100 volt mains. 



It is interesting to note that the oscillograph may be used to 

 show the power curves, as well as the curves 

 of current and potential difference. For this 

 purpose it is only necessary to excite the 

 electro-magnet by the alternating current, 

 instead of from a direct current supply, when 

 the instrument will act as a watt-meter, and 

 give the power-curves. The accuracy is not 

 so high when the oscillograph is used in this 

 way ; but the possibility of so using it is very 

 advantageous, both for purposes of demon- 

 stration and research. 



In order to observe the actual shape of the 

 wave-form, it is necessary to introduce a 

 movement of the beam of light reflected from 

 the oscillograph mirror at right angles to the 

 direction of vibration it already possesses. 

 This may be done by observing the movements of the spot 

 of light in a rotating mirror, or, if permanent records are 

 •desired, by receiving the spot on a moving photographic 

 plate or film. This method may be used for observing or 



between the oscillograph and the film which enables any length 

 of film up to 40 cm. to be used at each experiment, and this 

 shutter carries a contact maker which can be used to start any 

 non-periodic phenomena it may be desired to record. For the 

 cases in which it is only periodic changes which have to be 

 studied, Mr. Duddell has devised a very convenient arrangement 

 in which the rotating mirror is replaced by one which is vibrated 

 in synchronism with the waves of potential difference or current 

 under observation. The mirror is vibrated by a small syn- 

 chronous motor, and the spot of light is reflected from it on to 

 a transparent screen above it ; as the mirror moves with a uni- 

 formly increasing displacement the wave form is drawn on this 

 screen, and when arrived at the full extent of its motion, the mirror 

 is pressed back suddenly to its initial po.sition, the beam of light 

 being interrupted during this return journey by a shutter attached 

 to the motor shaft. The mirror then starts on a fresh swing, 

 and draws a second wave-form on the top of the first, the suc- 

 cessive curves appearing, by persistence of vision, on the screen 

 as a single bright line, which may be either traced, or photo- 

 graphed on sensitive paper. The size of the curves is about 

 3 cm. amplitude on each side of the zero and about 8 cm. in 

 length, one and a half complete wave-forms being shown in this 

 length. Fig 5 shows the synchronous motor in conjunction with 

 the large oscillograph, the photograph illustrating the complete 

 apparatus for projecting the curves on to a wall screen. On the 

 right is the source of light, and the oscillograph is on the extreme 



Fig. 6. — Mr. Duddell's portable pattern oscillograph. 



recording any variations in current, whether they be periodic 

 or not. In Fig. 4 is shown a photograph of the recording 

 arrangement. The oscillograph is in the back part of the box, 

 and in the front part may be seen the recording dium, which 

 carries Kodak daylight changing spools. There is a shutter 



Fig. 5. — Arrangement of apparatus for projection work. 



left ; to the right of the oscillograph is the motor which drives 

 the synchronous mirror, a view of the top of which is seen re- 

 flected in the glass above, this additional mirror being simply 

 introduced to reflect the beam of light, which comes vertically 

 from the synchronous mirror on the motor, horizontally on to the 

 wall. The apparatus will give wave-forms having an amplitude 

 of 50 cm. on either side of the zero and about 150 cm. long, show- 

 ing in this length one and a half wave-lengths, and sufficiently 

 bright to be seen by over two hundred people at one time. 



Mr. Duddell has, we understand, just completed a portable 

 form of oscillograph in which the electro-magnet is replaced by 

 a permanent magnet. This instrument has only one loop of 

 strip, so that it will only show one curve at a time ; the free 

 periodic time is 1/5000 of a second, and the sensibility at 1000 

 mm. scale distance is 750 mm. per ampere. Fig. 6 is a photo- 

 graph of this small oscillograph, and shows the instrument 

 with and without the front which protects all the working parts. 

 The whole apparatus for observing wave-forms — oscillograph, 

 source of light and rotating mirror — is fitted up ready for use in a 

 small and easily portable box, and should prove of great value 

 to central station engineers and others who employ alternating 

 currents. 



In Fig. 7 are shown some examples of curves of current 

 and potential difference obtained by means of the oscillo- 

 graph. These curves were photographed on to a falling 

 plate, and are here reproduced half full size. Curve i shows 

 the wave-forms of the P.D. between the terminals of a 

 dynamo sending a current through an inductive and non- 

 inductive resistance in series, and of the P.D. between the ter- 

 minals of the non-inductive part, as well as the wave-form of the 

 current flowing in the circuit. It will be seen that the self- 

 induction has caused the current curve to be out of phase with 

 the dynamo P.D., but there is no distortion of the shape. Curve 

 2 shows the characteristic wave-forms of the current and P.D. 

 of an alternating current arc, burning between solid carbons. 

 The P.D. has a high peak at the beginning, and the current 

 curve lies flat along the zero line at the beginning and end of 

 each half-wave. Curve 3 is for an arc burning between carbon 



NO. 1623, VOL. bl\ 



