The motion picture projectionist (Oct 1927-Sept 1928)

September, 1928 The Motion Picture Projectionist Use of Tachometers # By Nicholas M. Trapnell SPEED indicators, when first used on projectors in motion picture theatres, were considered a luxury, and were only found in a few of the best first-run houses. The advantages to be gained by the use of accurate speed indicators, or tachometers, soon became apparent, however, and today the projectors in nearly every first-run house are equipped with them. It is only by the use of tachometers that the modern theater is able to maintain an exact schedule, that the projectionist is able to run his projector at a definite, constant speed which will give the best results on the screen, and that the orchestra leader is able to correctly time his score ; but it is not only on projectors that tachometers have made possible improvements in technique which would otherwise have been impossible. Tachometers are now in general use, or are coming into general use, in other phases of motion picture production and exploitation, such as on cameras, where the importance of a standardized constant taking speed has long been recognized and insisted upon ; on film developing and printing machine and where this time must be varied with changes in the strength of the solutions ; and more recently, in the two systems of "talking motion pictures," the Vitaphone and Movietone, where the sound recording device must be perfectly synchronized with the camera. Tachometer a Necessity There are probably other phases of the motion picture industry where tachometers are now considered a luxury but where they will soon become a necessity, also there are probably phases where tachometers are not used at present, but where much could be gained by using them. My object is to discuss present uses of tachometers and to get suggestions and information on new applications of the tachometer in the motion picture field. There are certain general conditions which must be met by the tachometer manufacturer in nearly all applications of tachometers to motion picture work, and they may be briefly described as follows : The tachometer must be light and compact. This is especially important on all portable equipment, such as cameras, where any increase in weight or bulk is undesirable. It must require only a very small amount of power to drive it, because on most motion picture equipment there is very little surplus power available for driving the tachometer. This is true on motor-driven as well as hand-driven equipment, since the size of the motors is usually kept to a minimum. The accuracy must remain constant and must not be affected by wear of the me *S. M. P. E. Transactions. chanical parts. A tachometer whose accuracy decreases with increased wear of its parts is worse than useless after a short time in service. It must be reliable and require practically no attention, because most projectionists and cameramen have not had special training necessary to enable them to make repairs on tachometer equipment, if they had the time. The tachometer readings must often be transmitted to a point remote from the machine whose speed is being measured, as on projectors or remotely controlled cameras, and it is often necessary to have more than one indicator connected to the same machine, as on projectors in theatres where one indicator is mounted in the projection-room, one in the orchestra pit and sometimes a third in the manager's office. The tachometer equipment should be so designed that this can be done easily and cheaply. Types of Tachometers We now come to a consideration of the aifferent types, or classes of tachometer equipment available, and the degree to which they meet the above-mentioned conditions, which is a measure of their suitability for motion picture work. Tachometers may be divided into four general types or classes, according to the principles on which they operate. First — The mechanical type, which usually consists of a centrifugal device having a rotating mass whose position is dependent on the speed at which it is rotated and which is mechanically connected to a pointer moving over a scale calibrated in revolutions per minute, or other suitable units. Second — The hydraulic type, consisting of a small rotary pump which circulates liquid, usually an oil or glycerine, through a closed circuit in which is an orifice of definite diameter. The pressure of the liquid in the circuit between the pump discharge and the orifice is directly proportional to the speed at which the pump is driven and the speed is measured by means of a pressure gauge calibrated in revolutions per minute. Third — The magnetic drag type, in which a rotating permanent magnet tends to deflect an armature hung on pivots. The deflection of the armature is resisted by a spiral spring and the amount of deflection is proportional to the speed at which the magnet is rotated. The armature carries a scale which indicates, the speed of rotation. Fourth — The electric type, which consists of a small direct-current generator or magneto, driven from the device whose speed is to be measured, generating a voltage directly proportional to its speed, and connected by means of wires to a voltmeter calibrated to read in revolutions per minute, feet per minute, or other suitable units. The mechanical type of tachometer, although it can be built in a very light and compact form, is usually far from being accurate in its reading, due to wear of its parts ; and it is impossible to transmit its readings to a point remote from the machine whose speed is being measured. In spite of these defects, however, it is used to some extent in motion picture work. The hydraulic type, on account of its large size and weight, and the large amount of power necessary to drive it, is never used in motion picture work. The magnetic drag type is very little used in this work because it is usually necessary to drive it by means of a flexible shaft which consumes considerable power, is subject to wear, and adds materially to the weight and inconvenience of portable equipment. Electric Type Is Ideal The electric type is by far the most suitable tachometer for most motion picture work. It can be made very light and compact, its accuracy is not affected by wear of its mechanical parts, it requires an extremely small amount of power to drive it, and when properly designed and constructed requires no attention of any kind after installation. Its readings can be easily transmitted to any distance by extending the connecting wires between the magneto and indicator ; and as many indicators as desired can be run from the same magneto. There are now two classes of electric tachometers available for motion picture work. In the older class the magneto generates a very low potential, about eighttenths of a volt per 1,000 revolutions per minute, has a high internal resistance, about 70 ohms, a low output, and a low resistance in the external circuit, which includes the indicator and connecting leads. This class has several disadvantages. It is subject to error due to changes in resistance of the external circuit which may be caused by faulty electrical contacts or connections, or extreme temperature changes. Because of the likelihood of error due to resistance changes, the brushes which bear on the commutator of the magneto are made of a soft non-corrosive alloy to prevent corrosion at this contact point, and since this is a poor bearing metal, the brushes soon wear out. The magneto, indicator and connecting leads must all be adjusted and calibrated together and are not interchangeable. The indicators, because of the low output of the magneto, must be very sensitive which makes them delicate and subject to damage due to vibration, etc. This condition is aggra(Continued on page 26)