In criminal trials, in the courts of the United States, wherever a question arises whether a confession is incompetent because not voluntary, the issue is controlled by that portion of the Fifth Amendment to the constitution of the United States commanding that no person 'shall be compelled in any criminal case to be a witness against himself.' The legal principle by which the admissibility of the confession of an accused person is to be determined is expressed in the textbooks. In 3 Russell on Crimes (6th Ed.) 478, it is stated as follows: 'But a confession, in order to be admissible, must be free and voluntary; that is, must not be extracted by any sort of threats or violence, nor obtained by any direct or implied promises, however slight, nor by the exertion of any improper influence. . . . A confession can never be received in evidence where the prisoner has been influenced by any threat or promise; for the law cannot measure the force of the influence used, or decide upon its effect upon the mind of the prisoner, and therefore excludes the declaration if any degree of influence has been exerted.'
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