How does a water dowser work?

http://skepdic.com/dowsing.html

dowsing (a.k.a. water witching)

Dowsing is the action of a person--called the dowser--using a rod, stick or other device--called a dowsing rod, dowsing stick, doodlebug (when used to locate oil) or divining rod--to locate such things as underground water, hidden metal, buried treasure, oil, lost persons or golf balls, etc. Since dowsing is not based upon any known scientific or empirical laws or forces of nature, it should be considered a type of divination and an example of magical thinking. The dowser tries to locate objects by occult means.

Map dowsers use a dowsing device, usually a pendulum, over maps to locate oil, minerals, persons, water, etc. However, the prototype of a dowser is the field dowser who walks around an area using a forked stick to locate underground water. When above water, the rod points downward. (Some dowsers use two rods. The rods cross when above water.) Various theories have been given as to what causes the rods to move: electromagnetic or other subtle geological forces, suggestion from others or from geophysical observations, ESP and other paranormal explanations, etc. Most skeptics accept the explanation of William Carpenter (1852). The rod moves due to involuntary motor behavior, which Carpenter dubbed ideomotor action.

In the 16th century, Agricola described mining dowsers using a forked twig to find metals (De re metallica). He didn't think much of the practice. A miner, he wrote:

should not make us of an enchanted twig, because if he is prudent and skilled in the natural signs, he understands that a forked stick is of no use to him, for ... there are natural indications of the veins which he can see for himself without the help of twigs. (Quoted in Zusne and Jones 1989: 106)

Despite this sage advice, dowsers continue to dowse, claiming that they have a special power and that what they are dowsing for emanates energy, rays, radiations, vibrations, and the like.

Does dowsing work?

Some people are less interested in why the rods move than in whether dowsing works. Obviously, many people believe it does. Dowsing and other forms of divination have been around for thousands of years. There are large societies of dowsers in America and Europe and dowsers practice their art every day in all parts of the world. There have even been scientists in recent years who have offered proof that dowsing works. There must be something to it, then, or so it seems.

Testing has been sparse, however. For one thing, it is difficult to establish a "baseline against which a diviner's performance may be compared" (Zusne and Jones 1989: 108). In 1949, an experiment was conducted in Maine by the American Society for Psychical Research. Twenty-seven dowsers "failed completely to estimate either the depth or the amount of water to be found in a field free of surface clues to water, whereas a geologist and an engineer successfully predicted the depth at which water would be found in 16 sites in the same field...." (Zusne and Jones 1989: 108; reported in Vogt and Hyman: 1967). There have been a few other controlled tests of dowsing and all produced only chance results (ibid.). [In addition to Vogt and Hyman, see R.A. Foulkes (1971) "Dowsing experiments," Nature, 229, pp.163-168); M. Martin (1983-1984). "A new controlled dowsing experiment." Skeptical Inquirer. 8(2), 138-140; J. Randi(1979). "A controlled test of dowsing abilities." Skeptical Inquirer. 4(1). 16-20; and D. Smith (1982). "Two tests of divining in Australia." Skeptical Inquirer. 4(4). 34-37.]

The testimonials of dowsers and those who observe them provide the main evidence for dowsing. The evidence is simple: dowsers find what they are dowsing for and they do this many times. What more proof of dowsing is needed? The fact that this pattern of dowsing and finding something occurs repeatedly leads many dowsers and their advocates to make the causal connection between dowsing and finding water, oil, minerals, golf balls, etc. This type of fallacious reasoning is known as post hoc reasoning and is a very common basis for belief in paranormal powers. It is essentially unscientific and invalid. Scientific thinking includes being constantly vigilant against self-deception and being careful not to rely upon insight or intuition in place of rigorous and precise empirical testing of theoretical and causal claims. Every controlled study of dowsers, including the "Scheunen" or Barn study [see below], has shown that dowsers do no better than chance in finding what they are looking for.

Most dowsers do not consider it important to doubt their dowsing powers or to wonder if they are self-deceived. They never consider doing a controlled scientific test of their powers. They think that the fact that they have been successful over the years at dowsing is proof enough. When dowsers are scientifically tested and fail, they generally react with genuine surprise. Typical is what happened when James Randi tested some dowsers using a protocol they all agreed upon. If they could locate water in underground pipes at an 80% success rate they would get $10,000 (now the prize is over $1,000,000). All the dowsers failed the test, though each claimed to be highly successful in finding water using a variety of non-scientific instruments, including a pendulum. Says Randi, "the sad fact is that dowsers are no better at finding water than anyone else. Drill a well almost anywhere in an area where water is geologically possible, and you will find it."

Some of the strongest evidence for dowsing comes from Germany and the so-called "Scheunen" or "Barn" experiment. In 1987 and 1988, more than 500 dowsers participated in more than 10,000 double-blind tests set up by physicists in a barn near Munich. (Scheune is the German word for barn.) The researchers claim they empirically proved "a real dowsing phenomenon." Jim Enright of the Scripps Institute of Oceanography evaluated the data and concluded that the so-called "real dowsing phenomenon" can reasonably be attributed to chance. His argument is rather lengthy, but here is a taste of it:

The long and the short of it is that dowsing performance in the Scheunen experiments was not reproducible. It was not reproducible inter-individually: from a pool of some 500 self-proclaimed dowsers, the researchers selected for their critical experiments 43 candidates whom they considered most promising on the basis of preliminary testing; but the investigators themselves ended up being impressed with only a few of the performances of only a small handful from that select group. And, even more troublesome for the hypothesis, dowsing performance was not reproducible intra-individually: those few dowsers, who on one occasion or another seemed to do relatively well, were in their other comparable test series usually no more successful than the rest of the "unskilled" dowsers (Enright