The joule is a unit of _energy_ and can be thought of as the total capacity to do work. One joule is equivalent to 1 watt-second, or 1 watt applied for 1 second. But it can also mean a 10-watt burst of laser light lasting 0.1 second, or a billion watts lasting a billionth of a second. In general, the crystal (ruby) lasers are the most powerful, although other recently introduced materials, such as liquids (see Figure 27) and specially prepared glass, are providing competition. With proper auxiliary equipment, bursts of several _billion_ watts have been achieved; but the burst lasts only about 100 millionths of a second. For certain uses, that’s just what is wanted: a highly concentrated burst of energy that does its work without giving the material being “shot” a chance to heat up and spread the energy, perhaps damaging adjacent areas. [Illustration: Figure 27 _Active substance for a modern liquid laser is made in an uncomplicated 10-minute procedure. Bluish powder of the rare earth, neodymium, is dissolved in a solution of selenium oxychloride and sealed in a glass tube._] Since the joule gives a measure of the total energy in a laser burst it is not applicable to CW output. Power in this area began low—in the milliwatt (one thousandth of a watt) region—but has been creeping up steadily. A recent gas laser utilizing carbon dioxide has already reached 550 watts of continuous infrared radiation. This is the giant 44-footer shown in Figure 28. An advantage of gas (and liquid) lasers is that they can be made just about as large as one wishes. By way of comparison, the smallest gas laser in use is shown in Figure 29. [Illustration: Figure 28 _A giant 44-foot gas laser produces 550 watts of continuous power and is expected to reach 1000 watts. Glowing of the tube comes from gas discharge, not from laser light, which is in the infrared region and cannot be seen._] One of the least satisfactory aspects of the laser has been its notoriously low efficiency. For a while the best that could be accomplished was about 1%. That is, a hundred watts of light had to be put in to get 1 watt of coherent light out. In gas lasers the efficiency was even lower, ranging from 0.01% to 0.1%. In gas lasers this was no great problem since high power was not the objective. But with the high-power solid lasers, pumping power could be a major undertaking. A high-power laser pump built by Westinghouse Research Laboratories handles 70,000 joules. In more familiar terms, the peak power input while the pump is on is about 100,000,000 watts. For a brief instant this is roughly equal to all the electrical power needs of a city of 100,000 people.