in which the electric lead to the heater was replaced by a cord that took the weight of the heater, and the heater was heated by plugging into a socket in the chandelier. Outwardly, the machine looked similar to the earlier model, but at no time was there an electric potential near the head. Also during this time, in the United States a machine-less method that applied preheated clamps over the wrapped rods was invented, In 1931, at the Midwest Beauty Show in Chicago, Ralph L. Evans and Everett G. McDonough showed a heatless system for the first time. Their method used bi-sulfide solution and was often applied at the salon, left on while the client went home and removed the next day, leading it to be called the overnight wave. With the outbreak of World War II in 1939, all production of such equipment stopped in Europe and hardly recovered afterwards, being replaced either by home heater kits or cold-waving methods. Modern perms Cold wave In 1938, Arnold F. Willatt invented the cold wave, the precursor to the modern perm. It used no machines and no heat. The hair was wrapped on rods and a reduction lotion containing ammonium thioglycolate was applied. This chemical breaks open the disulfide linkages between the polypeptide bonds in the keratin; the protein structure in the hair. The disulfide bonds give hair its elasticity, and can be reformed with chemicals. Next, an oxidation lotion—hydrogen peroxide—was applied to close the disulfide bridges again, and the hair was reformed to the shape of the rod. The entire process took 6–8 hours at room temperature. In the 1970s, acid perms were invented. These ammonia-free acid perms, sometimes called buffered waves, use glyceryl monothioglycolate in the place of ammonia. Acid perms, or buffered wavers, are slower but gentler to the hair. Heat is usually added by placing the client under a dryer, after covering the wrapped head with a plastic cap. The reaction is endothermic and the additional heat causes the pH to rise from 6.9 to 7.2. Perms today use this method with sodium thioglycolate instead of ammonium thioglycolate, at a pH of 8 to 9.5. Perming hair with the sodium thioglycolate method takes only 15–30 minutes of exposure to the sodium thioglycolate solution before a neutralizer solution is applied to bring the pH back to normal and re-bond the hair. The permanent relaxer straightens the hair