Motion Picture News (Jul-Aug 1916)

1266 ACCESSORY NEWS SECTION Vol. 14. No. 8 The A B C of the Motion Picture Operating Room IT has often been said that nothing is known of electricity. This is contradictory, inasmuch as it is both true and false. We do not know what electricity is nor anything of its ultimate nature, but we do know a great deal about the laws which govern the action of electricity. The ordinary statement that an electric current is flowing along a wire is only a conventional way of expressing the fact that the wire and the space around the wire are in a different state from that in which they are when no electric current is said to be flowing. In order to make the average person understand the action of this so-called current, it is generally compared with the flow of water. In comparing hydraulics and electricity, it must be borne in mind, however, that there is really no such thing as an " electric fluid," and that water in pipes has mass and weight, while electricity has none. Whatever electricity may be, it is not matter and it is not energy. We do not speak of water as energy, but we do mean by a water power a quantity of wat^r under a head or pressure ; so a quantity of electricity under a pressure is stored energy, and can do work, for when water flows from a higher to a lower level, it is capable of doing work by driving some kind of a water wheel. The greater the height through which the water falls, the greater the amount of work it can do. The same thing is true of electricity. The greater the difference in electrical level, or difference of potential, or the greater the voltage, the greater the amount of electrical work the electricity can do. Unit of Difference Is the Volt The unit of difference of electrical level is the volt, and we may say that the volt corresponds to one foot of " head " in a system for developing power by water. The amount of power that can be developed from a. water fall depends on two things ; first, the fall in feet or the head, and second, the size of the stream. At Niagara Falls the power that can be developed is almost unlimited, not because the height of the fall is so great, but because the size of the stream is so great. Any water fall is capable of developing power, depending on the size of the stream. The unit of flow may be taken as one gallon per second. The corresponding quantity in electricity is called current, and the unit of current is called the ampere. The f.mpere then corresponds to the one gallon per second in a flow of water. The amount of water that can flow from a higher to a lower level depends on the size of the pipe line through which the water is led. For instance, if a pond of water fifty feet above the sea level were to discharge its contents through two pipe lines, one a six-inch pipe, the other a two-inch pipe, the quantity flowing through the six-inch pipe would be much greater than that which would flow through the pipe which only measured two inches. Again, if there are two pipes of the same size leading from the pond to the sea and one is twice as long as the other, about twice as much water will flow through the short pipe as through the longer one. The friction of the water is greater in the long pipe, or the resistance to the flow is greater, and so less water flows. Unit of Resistance Well Defined There is no convenient unit for the resistance of a pipe to the flow of water, but the unit of resistance of a wire carrying an electrical current is well defined and is called the ohm. Remember, please, that a pipe of small diameter offers a greater resistance to the flow of water than a pipe of larger diameter. So a wire of small diameter offers more resistance to an electric current than a wire of large diameter. If we double the cross-section of a wire, we cut its resistance in half, but if we double the length of the wire, we double its resistance. If we double the cross-section and double the length of the wire the resistance remains the same. This law may be expressed thus : For a wire of a given substance the resistance is directly proportional to the length, and inversely proportional to the cross-section. A thorough understanding of Ohm's law is of invaluable aid to the operator, for by possessing this knowledge he will be able to figure out for himself many problems that would be almost impossible for him to do otherwise. Now, we find that discharge or current equals head or pressure divided by friction or resistance. In a circuit carrying electricity the sam? thing is true and we have : Electrical discharge or current equals electrical head or pressure, divided by electrical friction or resistance. Since the unit of electrical current is the ampere, and the unit of pressure is the volt, and the unit of resistance is the ohm, we have : Amperes equal volts divided by ohms, or to make it more clear we will put it in the form of a fraction, and have: (E.M.F.) Volts (C) = Amperes equals (R) Ohms The Value of Ohm's Law This is where we get to understand the value of Ohm's law. Since the electrical pressure is what causes the movement of the electrical current, it is called the electro-motive force, and as this term is very long it is abbreviated to E.M.F. Since the amperes measure the amount of flow of electricity, this flow is called current, and is abbreviated to C. Resistance is abbreviated to R., and thus we have our Ohm's law. We now know that if we divide the voltage by the ohms we will be able to figure out the amount of current (amperes) we are using, but to figure out the voltage and resistance, knowing the amperage is a different matter and seems to puzzle a number of operators when it really should not. It is quite simple, and it will only require a few minutes study to be able to master it completely, or what is better, make a drawing on cardboard like V this : , and when you want to find anv one quantity, cover AO up that symbol or letter, and what remains will be the answer. For instance, supposing you knew the amperage and voltage, and wanted to know the amount of resistance to use to give you the proper voltage at the arc. By covering up the " O," you will see that the resistance is found by dividing the voltage by the amperage ; likewise, if we want to find the amount of current we are using we cover the " A " and divide voltage by ohms which gives us the amperage. I do not claim to be the originator of this method, although I have used it for many years and found it to be of great assistance. In the formula, " V " represents the voltage ; " A " the amperage, and " O '' the resistance. To make my meaning more clear, we will suppose that we have a 110-volt circuit, and are using a rheostat of five ohms resistance, and we desire to know the number of amperes being delivered. Bj' using the formula, and substituting figures for svmbols we proceed as follows ; V 110 C = — = = 22 amperes O 5 The answer would be 22 amperes, but this is only approximately for there are other things to be taken into consideration, such as wire and carbon resistance as well as the resistance of the arc itself, there is a high resistance offered to the current when it is passing from one carbon to the other and this resistance is made greater or lesser by the way in which the lamp is handled or in other words, the length of arc : for when a long arc is being held the resistance is greater while with a short arc the resistance is less, for resistance falls when the current is increased, and rises when it is decreased. You will see by this that the arc resistance is not like a wire resistance, but rather to the contrary. The Watt Is the Unit of Power Now we come to that which interests the manager most, the watt. The watt is the unit of power, and it is advisable for the operator to be able to figure out just how much power he is using so that he can inform his employer as to the correctness of the lighting bills as rendered by the lighting company. A watt is 1-746 of a horse-power, and to find out the number of kilowatts, and the cost of same you would proceed as follows : On a voltage of 110, with an amperage of, say, 30, for convenience sake, you would multiply the voltage by the amperage, 110 x 30 equals 3,300, which divided by 100 gives as the kilowatts 3.3. Supposing you were operating your machines ten hours a day. you know that the 3.3 kilowatts is for one hour, so you multiply 3.3 by 10, which is 33 kilowatts per day. figuring the cost at 10 cents per kilowatt, the daily cost of current will be about $3.30. To find the cost for the week or month, as the case may be, multiply