Following an introductory review of the concept of macroscopic quantization, simple physical notions underlying the understanding of the properties of superconductors involving weak links is given. A brief treatment of the Josephson effect is given as an introduction to the application of macroscopic quantum coherence effects to scientific devices. Special emphasis is given to the description of devices involving double or single weak links for measurements of very small magnetic fields and voltages. Advantages and limitations of various methods is briefly described together with problems and possibilities involved in future application of the A. C. Josephson effect.
