December 6, 1900] 



NATURE 



14: 



tion. A small bar of soft iron, M, to which is attached a light 

 mirror, is pivoted between the pole-pieces, P.P., of a powerful 

 magnet or electro-magnet. These pole-pieces are laminated and 

 are specially shaped to give as strong a magnetic field in the air 

 gap as possible. On each side of the pole-pieces is a coil of 

 wire, B.B., through which the cijrrent to be observed flows. 

 This current produces a field at right angles to that of the field 

 magnet, and so deflects the iron bar through an angle which, if 

 small, is proportional to the current. M. Blondel has produced 

 an instrument of this type having a free periodic .time of 

 i/6oooth of a second ; and, by replacing the small bar with 

 an iron band stretched between the pole pieces, he has con- 

 structed an instrument having much smaller free periodic time 

 than any other type of oscillograph — indeed as small as 

 1/50, 000th of a second — but the sensibility at this high frequency 

 is not very great. At a frequency of about 10,000 vibrations 

 per second, its sensibility is about the same as thai of the other 

 types of instrument. 



The credit of developing the bifilar oscillograph is due to Mr. 

 Duddell, who, as the result of a long series of investigations 



Fig. 



-Mr. Duddell's oscillograph. (From the Journal of the Inst. Elect. 

 Eng., vol. xxviif., 1899, p. 8.) 



carried out at the Central Technical College, has produced in- 

 struments of this type possessing a high degree of perfection, and 

 by means of their use has brought to light a number of new ex- 

 perimental facts. The principle of Mr. Duddell's oscillograph 

 will be easily understood by reference to Fig. 2. The current 

 to be observed flows up one side and down the other of the 

 continuous strip of phosphor-bronze, s.s.s.s. This strip is 

 looped over the pulley, P, which is attached to a small 

 spring balance (see Fig. 3) by means of which the tension 

 on the strip can be regulated. Each arm of the loop passes 

 through the gap between the poles, N.S. , of a powerful electro- 

 magnet. The loop carries at its centre a mirror, M, which is 

 made of a small piece of silvered cover-glass cemented to the 

 strips. When a current passes through the loop, one side is 

 moved forward and the other backward, and the mirror is thus 

 deflected through an angle proportional to the current. The 

 phosphor-bronze strips are held in position at the bottom by 

 being clamped between ebonite insulating pieces at K, and at 

 the top by being drawn against the single ebonite piece at L. It 

 will be observed, therefore, that the only part of the strip that 



NO. 1623, VOL. 63I 



takes part in the vibration is that between K and L, and not, as' 

 might otherwise be supposed, the whole length from K to the 

 pulley P. 



By the use of phosphor-bronze, Mr. Duddell has found it 

 possible to make very light strips having sufficient strength to 

 enable considerable tension to be used, and having, at the 

 same time, good conductivity. He has been able to bring 

 down the free periodic time to i/io,oooth of a second, and, with 

 a free periodic time as low as this, the mirror can easily follow, 

 with extreme accuracy, the vibrations of an electric current 

 alternating at the rate of 300 complete cycles per second, while 

 even if the alternating current has a periodic time as short as 

 O'ooi second, or even less, the record may possess sufficient ac- 

 curacy for many purposes. The narrow gap through which the 

 strips pass is only just large enough to allow of their free move- 

 ment, and as the break in the magnetic circuit is consequently 

 very small, the field can be made very intensf^. This space is 

 filled with damping-oil, which is retained in it by a lens forming 

 a front to the gap, and thus the strips are confined in a narrow 

 oil-bath, in which they have only just room to move, the 

 damping in consequence being very efficient and rendering the 

 instrument accurately dead-beat. 



Already the Cambridge Scientific Instrument Co. have con- 

 structed many specimens of two types of this form of oscillograph, 



Fig. 3. — Mr. Duddell's oscillograph. High frequency pattern. 



a large one for projection work and a high frequency instrument 

 for more accurate research work. The general design in both 

 types is the same, the chief difference lying in the high frequency 

 pattern having its moving parts smaller and lighter, by which 

 means the periodic time of i/io,oooth second has been obtained, 

 whereas the free periodic time of the projection instrument is 

 i/2000th of a second. The instrument is made with two loops 

 fixed side by side in the gap so that one may be used to give the 

 wave-form of the current while the other is used to give the 

 wave-form of the potential difference, the two curves being thus 

 obtained simultaneously. There is a third fixed mirror between 

 the two vibrating mirrors which is used to give a zero line. 

 Small tangent screws enable the positions of the moving 

 mirrors to be adjusted to zero. From Fig. 3, which is a photo- 

 graph of the double instrument, a good idea of its general 

 appearance and construction may be obtained. The light band 

 between the poles of the magnet shows the position of the 

 mirrors, but the illustration is on too small a scale for the n.irrors 

 themselves to be distinctly visible. This is not surprising when 

 one considers that their actual size is only I'O mm, high by 

 o'3 mm. wide by about o i mm. thick. The strips are connected 

 through the four small upright fuses with the terminals on the 

 front of the base of the instrument. The normal working current 

 in the strips is O'lO amp., and the sensibility at a scale distance 



