Generally, an invention is the property of the inventor who conceived, developed, and perfected it. Hence, the mere fact that the inventor was employed by another at the time of the invention does not mean that that inventor is required to assign the patent rights to the employer. The right, if any, of an employer to inventions of its employee is determined primarily by the contract of employment. If, as here, the contract of employment does not contain an express provision respecting the subject, an employer is, nonetheless, not necessarily precluded from claiming a right to the invention. If an employee's job duties include the responsibility for inventing or for solving a particular problem that requires invention, any invention created by that employee during the performance of those responsibilities belongs to the employer. Hence, such an employee is bound to assign to the employer all rights to the invention. This is so because, under these circumstances, the employee has produced only that which he was employed to produce, and the courts will find an implied contract obligation to assign any rights to the employer. United States v. Dubilier Condenser Corp., 289 U.S. 178, 53 S. Ct. 554, 77 L. Ed. 1114 (1933); Solomons v. United States, 137 U.S. 342, 11 S. Ct. 88, 34 L. Ed. 667 (1890); Hewett v. Samsonite Corp., 32 Colo. App. 150, 507 P.2d 1119 (1973). On the other hand, if an employee is not employed to invent or to solve a particular problem, that employee owns the right to any invention made by the employee during the term of employment. However, under such circumstances, if the employer has contributed to the development of the invention, such as by paying for the employee's efforts, the employer has a "shop right" to use it free of charge and without liability for infringement. United States v. Dubilier Condenser Corp., supra; Solomons v. United States, supra; Hewett v. Samsonite Corp., supra.